git-am: record full index line in the patch used while rebasing

Earlier, a230949 (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>
diff --git a/git-am.sh b/git-am.sh
index c682d34..0e0a096 100755
--- a/git-am.sh
+++ b/git-am.sh
@@ -664,7 +664,7 @@
 			sed -e '1,/^$/d' >"$dotest/msg-clean"
 			echo "$commit" >"$dotest/original-commit"
 			get_author_ident_from_commit "$commit" >"$dotest/author-script"
-			git diff-tree --root --binary "$commit" >"$dotest/patch"
+			git diff-tree --root --binary --full-index "$commit" >"$dotest/patch"
 		else
 			git mailinfo $keep $no_inbody_headers $scissors $utf8 "$dotest/msg" "$dotest/patch" \
 				<"$dotest/$msgnum" >"$dotest/info" ||