Merge branch 'xy/format-patch-base'

Merge branch 'xy/format-patch-base'

"git format-patch" learned a new "--base" option to record what
(public, well-known) commit the original series was built on in
its output.

* xy/format-patch-base:
  format-patch: introduce format.useAutoBase configuration
  format-patch: introduce --base=auto option
  format-patch: add '--base' option to record base tree info
  patch-ids: make commit_patch_id() a public helper function