Getting git to run your diff and merge tools for you, instead of struggling with what compare, can save you immense amounts of time.
git merge another-branch
# there is a conflict, oh noes!
git mergetool[diff]
tool = bcomp
[difftool]
prompt = false
[difftool "meld"]
cmd = /Applications/Meld.app/Contents/MacOS/Meld \"$LOCAL\" \"$REMOTE\"
[difftool "bcomp"]
cmd = bcomp \"$LOCAL\" \"$REMOTE\"
[difftool "sourcetree"]
cmd = opendiff \"$LOCAL\" \"$REMOTE\"
[merge]
tool = bcomp
[mergetool]
prompt = false
[mergetool "meld"]
trustExitCode = false
cmd = /Applications/Meld.app/Contents/MacOS/Meld \"$LOCAL\" \"$MERGED\" \"$REMOTE\"--output=\"$MERGED\"
[mergetool "bcomp"]
trustExitCode = false
cmd = bcomp \"$LOCAL\" \"$REMOTE\" \"$MERGED\" \"$MERGED\"
[mergetool "sourcetree"]
trustExitCode = false
cmd = /Applications/Sourcetree.app/Contents/Resources/opendiff-w.sh \"$LOCAL\" \"$REMOTE\" -ancestor \"$BASE\" -merge \"$MERGED\"