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 <matled@gmx.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

setup.c

@@ -184,6 +184,10 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		}
 		return NULL;
 	bad_dir_environ:
+		if (!nongit_ok) {
+			*nongit_ok = 1;
+			return NULL;
+		}
 		path[len] = 0;
 		die("Not a git repository: '%s'", path);
 	}