git p4 clone --bare: need to be explicit about the gitdir

When `safe.bareRepository` will change to be safe by default, bare
repositories won't be discovered by default anymore. To prepare for
this, `git p4` must be explicit about the gitdir when cloning into a
bare repository, and no longer rely on that implicit discovery.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -4360,6 +4360,7 @@ class P4Clone(P4Sync):
         init_cmd = ["git", "init"]
         if self.cloneBare:
             init_cmd.append("--bare")
+            os.environ["GIT_DIR"] = os.getcwd()
         retcode = subprocess.call(init_cmd)
         if retcode:
             raise subprocess.CalledProcessError(retcode, init_cmd)