Merge branch 'bc/format-patch-cover-no-attach'

"git format-patch --cover --attach" created a broken MIME multipart
message for the cover letter, which has been fixed by keeping the
cover letter as plain text file.

* bc/format-patch-cover-no-attach:
  format-patch: make cover letters always text/plain

diff --cc builtin/log.c
Simple merge

diff --cc log-tree.c
Simple merge