Merge branch 'fc/send-email-annotate'

Allows format-patch --cover-letter to be configurable; the most
notable is the "auto" mode to create cover-letter only for multi
patch series.

* fc/send-email-annotate:
  rebase-am: explicitly disable cover-letter
  format-patch: trivial cleanups
  format-patch: add format.coverLetter configuration variable
  log: update to OPT_BOOL
  format-patch: refactor branch name calculation
  format-patch: improve head calculation for cover-letter
  send-email: make annotate configurable

diff --cc Documentation/config.txt
Simple merge

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