I just did this recently and its hella useful…

# Configure textmate globally as the commit comment editor for git

$ git-config –global core.editor ‘mate -w’

# Configure opendiff(aka FileMerge) as the git merge tool

$ git config –global merge.tool opendiff

via yawningman: useful git stuff on OSX.

UPDATE: my git version (1.6.0.2) accepts a slightly different syntax:

git config –global core.editor “mate -w”