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

diff --cc Documentation/config.txt
Simple merge

diff --cc builtin/log.c
Simple merge

diff --cc t/t4014-format-patch.sh
Simple merge