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

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>