Merge branch 'jt/use-trailer-api-in-commands' into next

Merge branch 'jt/use-trailer-api-in-commands' into next

Commands that operate on a log message and add lines to the trailer
blocks, such as "format-patch -s", "cherry-pick (-x|-s)", and
"commit -s", have been taught to use the logic of and share the
code with "git interpret-trailer".

* jt/use-trailer-api-in-commands:
  sequencer: use trailer's trailer layout
  trailer: have function to describe trailer layout
  trailer: avoid unnecessary splitting on lines
  commit: make ignore_non_trailer take buf/len
  trailer: be stricter in parsing separators