receive-pack: Initialize PATH to include exec-dir.

511707d (use only the $PATH for exec'ing git commands) made it a
requirement to call setup_path() to include the git exec-dir in PATH
before spawning any other git commands. git-receive-pack was not yet
adapted to do this and therefore fails to spawn git-unpack-objects if that
is not in the standard PATH.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

receive-pack.c

@@ -469,6 +469,8 @@ int main(int argc, char **argv)
 	if (!dir)
 		usage(receive_pack_usage);
 
+	setup_path(NULL);
+
 	if (!enter_repo(dir, 0))
 		die("'%s': unable to chdir or not a git archive", dir);