Earlier, a2309494

 (am --rebasing: get patch body from commit, not
from mailbox, 2012-06-26) learned to regenerate patch body from the
commit object while rebasing, instead of reading from the rebase-am
front-end.  While doing so, it used "git diff-tree" but without
giving it the "--full-index" option.

This does not matter for in-repository objects; during rebasing, any
abbreviated object name should uniquely identify them.

But we may be rebasing a commit that contains a change to a gitlink,
in which case we usually should not have the object (it names a
commit in the submodule).  A full object name is necessary to later
reconstruct a fake ancestor index for them.

Signed-off-by: Junio C Hamano <gitster@pobox.com>