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I'm just starting to work with Git. I would like to use TortoiseMerge as the difftool and mergetool.

In my $HOME/.gitconfig I have the following sections. I've removed the user and color sections for this question.

[merge]
    tool = tortoisemerge
[mergetool "tortoisemerge"]
    cmd = \"TortoiseMerge.exe\" -base:\"$BASE\" -mine:\"$LOCAL\" -theirs:\"$REMOTE\" -merged:\"$MERGED\"
[diff]
    tool = tortoisemerge
[difftool "tortoisemerge"]
    cmd = \"TortoiseMerge.exe\" -base:\"$BASE\" -mine:\"$LOCAL\" -theirs:\"$REMOTE\" -merged:\"$MERGED\"
If I type tortoisemerge at the Git Bash prompt it loads. It is known to be on the path. But if I type the command I get the following error.
Rich:mygittest (master *)
$ git difftool
error: 'tortoisemerge' can only be used to resolve merges
merge tool candidates: kompare emerge vimdiff
No known merge resolution program available.
external diff died, stopping at readme.txt.
Rich:mygittest (master *)
What am I not understanding to make this work?
Tortoisemerge is installed with TortoiseSVN.
The following settings work fine for me. However, I am using TortoiseGit not TortoiseSVN. Notice the difference in the parameters for diff.
[diff]
  tool = tortoisediff
[difftool]
  prompt = false
[merge]
  tool = tortoisemerge
[mergetool]
  prompt = false
  keepBackup = false
[difftool "tortoisediff"]
  cmd = \""c:/Program Files/TortoiseGIT/bin/TortoiseGitMerge.exe"\" -mine "$REMOTE" -base "$LOCAL"
[mergetool "tortoisemerge"]
  cmd = \""c:/Program Files/TortoiseGIT/bin/TortoiseGitMerge.exe"\" -base "$BASE" -theirs "$REMOTE" -mine "$LOCAL" -merged "$MERGED"
                TortoiseMerge has an issue with the way Git mangles the parameters. Because TortoiseMerge uses a colon to separate parameter name from value, Git puts quotes around both when dealing with filenames with spaces. TortoiseGitMerge allows a space and relieves the problem.
– Rich Shealer
                May 11, 2013 at 10:14
                Perforce can't cope with colons either. We should ask TortoiseMerge to add an alternative syntax.
– Colonel Panic
                Aug 15, 2014 at 9:13
                It works for me when having TortoiseSVN which has TortoiseMerge.exe. Yes, diff with TortoiseMerge.exe, not TortoiseIDiff.exe. git difftool calls out TortoiseMerge GUI, very convenient! TortoiseMerge is the best!
– WesternGun
                Nov 30, 2017 at 17:09
So that filenames with spaces are handled correctly, you should change the last line of @melbourn's answer to
cmd = \""c:/Program Files/TortoiseGIT/bin/TortoiseGitMerge.exe"\" -base "$BASE" -theirs "$REMOTE" -mine "$LOCAL" -merged "$MERGED"
                What's the difference?  Is it (just) that the colons in the parameters have been replaced by spaces?  Should a similar change be made for the difftool?  I don't know if it's worth editing the original answer, or whether there is some reason for the colons..
– mwfearnley
                Aug 27, 2014 at 22:33
                The colons are replaced with spaces. The argument handling code can accept either colons or spaces. But quotes around a filename which has spaces in it will not be handled correctly if the quotes are not at either the start or the end of 'words'.
– jwg
                Aug 31, 2014 at 7:05
This is a simpler way, just run these commands in git bash:
git config --global merge.tool tortoisemerge
git config --global diff.tool tortoisediff
git config --global difftool.tortoisediff.cmd "TortoiseGitMerge \$LOCAL \$REMOTE"
It's done.
----for futher reading----
There are other settings you may need:
# you may want to disable prompt or merge backup
git config --global mergetool.keepBackup false
git config --global difftool.prompt false
git config --global mergetool.prompt false
# set the path only when it's not found in system path
git config --global mergetool.tortoisemerge.path "C:\Program Files\TortoiseGit\bin\TortoiseGitMerge.exe" 
git config --global difftool.tortoisdiff.path "C:\Program Files\TortoiseGit\bin\TortoiseGitMerge.exe"
To set the mergetool, you just need to set the right mergetool name. It's official supported.
Git for windows installer includes a shell file for running tortoisegitmerge as a merge tool. It locates at C:\Program Files\Git\mingw64\libexec\git-core\mergetools. (Git provides 4 variables for merging: $BASE $LOCAL $REMOTE $MERGED, you can check them in the shell file.)
You can check all supported merge/diff tools supported on your machine with the command:
git difftool --tool-help
git mergetool --tool-help
Run tortoisegitmerge as git diff tool is not official supported by git, maybe it's because this tool doesn't support directory compare (git difftool -d).  So you need to config the difftool cmd to use it as a git difftool.
The tortoisegitmerge tool has a manual at:

https://tortoisegit.org/docs/tortoisegitmerge/tme-automation.html

You can find the simplest way to do diff file is:

TortoiseGitMerge BaseFilePath MyFilePath [ TheirFilePath ].

So I use this way for git difftool's command.
Great answer by Klas Mellbourn! Saved me a lotta time.
One shortcoming is, newly Added or Removed files in the repository will not show up during the difftool command execution. There's nothing to compare them to! Here's what I did on top of this:
(Inspired by my co-worker's answer).
Create a file named empty.empty in $Home directory (execute start ~ in your bash). And as the name suggests, keep it empty.
Create another file named tortoisediff.sh in $Home/bin directory with following content
#!/bin/sh
# $LOCAL $REMOTE seem to be swapped
# $1 is $LOCAL
# $2 is $REMOTE
difftool='/c/Program Files/TortoiseGit/bin/TortoiseGitMerge.exe'
NULL="/dev/null"
empty="C:/home/empty.empty"
if [ "$1" == "$NULL" ]; then
    echo "Added: " "$2"
    "$difftool" /base:"$empty" /mine:"$2" /readonly:"$empty"
elif [ "$2" == "$NULL" ]; then
    echo 'Removed: ' "$1"
    "$difftool" /base:"$1" /readonly:"$1" /mine:"$empty"
    echo 'Modified' "$2"
    "$difftool" /base:"$1" /basename:"$1" /readonly:"$1" /mine:"$2" /minename:"$2"
# Checkout https://tortoisegit.org/docs/tortoisegitmerge/tme-automation.html for more
Modify your .gitconfig file (Line 11 of the answer)
cmd = tortoisediff.sh "$LOCAL" "$REMOTE"
This would now make difftool refer to tortoisediff.sh instead of opening the application directly.
Keep in mind: you'd have to run git add . followed by git difftool --staged instead of simply git difftool.
  keepBackup = false
[difftool "tortoisediff"]
  cmd = \""C:/Users/$USER/Desktop/TortoiseMerge.exe"\" -base:\"$BASE\" -mine:\"$LOCAL\" -theirs:\"$REMOTE\" -merged:\"$MERGED\"
[mergetool "tortoisemerge"]
  cmd = \""C:/Users/$USER/Desktop/TortoiseMerge.exe"\" -base:\"$BASE\" -mine:\"$LOCAL\" -theirs:\"$REMOTE\" -merged:\"$MERGED\"
This worked for me, using TortoiseMerge 1.6.7 (Portable)
When rebasing, I strongly recommend switch $theirs and $mine, because it is different when you merge and rebase-merge. Check here:
What is the precise meaning of "ours" and "theirs" in git?
So, if you only use mergetool to rebase like me, do:
[mergetool "tortoisemerge"]
cmd = \""c:/Program Files/TortoiseGIT/bin/TortoiseGitMerge.exe"\" -base "$BASE" -theirs "$LOCAL" -mine "$REMOTE" -merged "$MERGED"
For using the TortoiseMerge with an existing TortoiseSVN installation in Windows:
[guitool "VisualDiff"]
    cmd = git difftool -y \"$FILENAME\"
    needsfile = yes
    noconsole = yes
[diff]
    tool = tortoisediff
[difftool]
    prompt = false
[merge]
    tool = tortoisemerge
[mergetool]
    prompt = false
    keepBackup = false
[difftool "tortoisediff"]
    cmd = \""W:/Programs/TortoiseSVN/bin/TortoiseMerge.exe"\" "/mine:$REMOTE" "/base:$LOCAL"
[mergetool "tortoisemerge"]
    cmd = \""W:/Programs/TortoiseSVN/bin/TortoiseMerge.exe"\" "/base:$BASE" "/theirs:$REMOTE" "/mine:$LOCAL" "/merged:$MERGED"
        
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