builtin/commit.c: extract ignore_non_trailer() helper function

Extract a helper function from prepare_to_commit() to determine
where to place a new Signed-off-by: line, which is essentially the
true "end" of the log message, ignoring the trailing "Conflicts:"
line and everything below it.

The detection _should_ make sure the "Conflicts:" line it finds is
truly the conflict hint block by checking everything that follows is
a HT indented pathname to avoid false positive, but this logic will
be revamped in a later patch to ignore comments and blanks anyway,
so it is left as-is in this step.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
merge & sequencer: turn "Conflicts:" hint into a comment

Just like other hints such as "Changes to be committed" we show in
the editor to remind the committer what paths were involved in the
resulting commit to help improving their log message, this section
is merely a reminder.

Traditionally, it was not made into comments primarily because it
has to be generated outside the wt-status infrastructure, and also
because it was meant as a bit stronger reminder than the others
(i.e. explaining how you resolved conflicts is much more important
than mentioning what you did to every paths involved in the commit).

But that still does not make this hint a part of the log message
proper, and not showing it as a comment is inviting mistakes.

Note that we still notice "Conflicts:" followed by list of indented
pathnames as an old-style cruft and insert a new Signed-off-by:
before it.  This is so that "commit --amend -s" adds the new S-o-b
at the right place when used on an older commit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>