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