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