pretty: split oneline and email subject printing

pretty: split oneline and email subject printing

The pp_title_line() function is used for two formats: the oneline format
and the subject line of the email format. But most of the logic in the
function does not make any sense for oneline; it is about special
formatting of email headers.

Lumping the two formats together made sense long ago in 4234a76167
(Extend --pretty=oneline to cover the first paragraph, 2007-06-11), when
there was a lot of manual logic to paste lines together. But later,
88c44735ab (pretty: factor out format_subject(), 2008-12-27) pulled that
logic into its own function.

We can implement the oneline format by just calling that one function.
This makes the intention of the code much more clear, as we know we only
need to worry about those extra email options when dealing with actual
email.

While the intent here is cleanup, it is possible to trigger these cases
in practice by running format-patch with an explicit --oneline option.
But if you did, the results are basically nonsense. For example, with
the preserve_subject flag:

  $ printf "%s\n" one two three | git commit --allow-empty -F -
  $ git format-patch -1 --stdout -k | grep ^Subject
  Subject: =?UTF-8?q?one=0Atwo=0Athree?=
  $ git format-patch -1 --stdout -k --oneline --no-signature
  2af7fbe one
  two
  three

Or with extra headers:

  $ git format-patch -1 --stdout --cc=me --oneline --no-signature
  2af7fbe one two three
  Cc: me
So I'd actually consider this to be an improvement, though you are
probably crazy to use other formats with format-patch in the first place
(arguably it should forbid non-email formats entirely, but that's a
bigger change).

As a bonus, it eliminates some pointless extra allocations for the
oneline output. The email code, since it has to deal with wrapping,
formats into an extra auxiliary buffer. The speedup is tiny, though like
"rev-list --no-abbrev --format=oneline" seems to improve by a consistent
1-2% for me.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>