There are a few programs, such as config and diff, which allow running
without a git repository.  Therefore, they have to call
setup_git_directory_gently().

However, when GIT_DIR and GIT_WORK_TREE were set, and the current
directory was a subdirectory of the work tree,
setup_git_directory_gently() would return a bogus NULL prefix.

This patch fixes that.

Noticed by REPLeffect on IRC.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>