Why can’t I use TortoiseMerge as my git merge tool on Windows?

If you have the latest git, run this command line once:

git config merge.tool tortoisemerge

Important: Do not add a .exe extension to the command.

If that fails, or if you want to add a different merge tool that git doesn’t know about, do this:

Open one of the following in an editor:

• 64-bit git: C:\Program Files\Git\mingw64\share\git-gui\lib\mergetool.tcl
• 32-bit git: C:\Program Files (x86)\Git\share\git-gui\lib\mergetool.tcl

Add something like this to mergetool.tcl:

tortoisemerge {
    set cmdline [list TortoiseMerge.exe -base:"$BASE" -mine:"$LOCAL" -theirs:"$REMOTE" -merged:"$MERGED"]
}

Put the new tortoisemerge entry above this other code:

default {
        error_popup [mc "Unsupported merge tool '%s'" $tool]
        return
}

Bonus example:

To use SourceGear diffmerge, add this to mergetool.tcl:

diffmerge {
    set cmdline [list "C:/Program Files (x86)/SourceGear/DiffMerge/DiffMerge.exe" --merge --result=$MERGED $LOCAL $BASE $REMOTE]
}