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