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>