patch-id: fix patch-id for mode changes

patch-id: fix patch-id for mode changes

Currently patch-id as used in rebase and cherry-pick does not account
for file modes if the file is modified. One consequence of this is
that if you have a local patch that changes modes, but upstream
has applied an outdated version of the patch that doesn't include
that mode change, "git rebase" will drop your local version of the
patch along with your mode changes. It also means that internal
patch-id doesn't produce the same output as the builtin, which does
account for mode changes due to them being part of diff output.

Fix by adding mode to the patch-id if it has changed, in the same
format that would be produced by diff, so that it is compatible
with builtin patch-id.

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