Merge branch 'nd/git-dir-pointing-at-gitfile'

We made sure that we notice the user-supplied GIT_DIR is actually a
gitfile, but did not do the same when the default ".git" is a gitfile.

* nd/git-dir-pointing-at-gitfile:
Make setup_git_env() resolve .git file when $GIT_DIR is not specified

diff --cc environment.c
Simple merge