setup_git_directory_gently: do not barf when GIT_DIR is given.
setup_git_directory_gently: do not barf when GIT_DIR is given.

Earlier we barfed when GIT_DIR environment variable points at a
directory yet to be created, which made it impossible to use
configuration mechanism in "git-init-db".

Signed-off-by: Matthias Lederhofer <507cbf130ec1f0ec254653b4a9393a0899dd8ba7@gmx.net>
Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git