Setting GIT_INDEX_FILE clobbers the rest of the environment, making git
not read ~/.gitconfig, and blow up if GECOS didn't have a name for the
user.
I'm not entirely happy with getEnvironment being run every time now,
that's somewhat expensive. It may make sense to just set GIT_COMMITTER_*
and GIT_AUTHOR_*, but I worry that clobbering the rest could break PATH,
or GIT_PATH, or something else that might be used by a command run in here.
And caching the environment is not a good idea either; it can change..
