How to setup UnityYAMLMerge.exe on Git for Windows
add "C:\Program Files\Unity\Editor\Data\Tools\" to your path in Windows

git config --global merge.tool unityyamlmerge
git config --global mergetool.unityyamlmerge.cmd 'unityyamlmerge.exe merge -p "$(cygpath -wla $BASE)" "$(cygpath -wla $LOCAL)" "$(cygpath -wla $REMOTE)" "$(cygpath -wla $MERGED)"'
git config --global mergetool.unityyamlmerge.trustExitCode false

This makes merging .scene files a breeze.