Merge branch 'sb/format-patch-base-patch-id-fix'

The "--base" option of "format-patch" computed the patch-ids for
prerequisite patches in an unstable way, which has been updated to
compute in a way that is compatible with "git patch-id --stable".

* sb/format-patch-base-patch-id-fix:
format-patch: make --base patch-id output stable
format-patch: inform user that patch-id generation is unstable

diff --cc diff.c
Simple merge

diff --cc diff.h
Simple merge