interpret-trailers: honor the cut line

interpret-trailers: honor the cut line

If a commit message is edited with the "verbose" option, the buffer
will have a cut line and diff after the log message, like so:

    my subject

    # ------------------------ >8 ------------------------
    # Do not touch the line above.
    # Everything below will be removed.
    diff --git a/foo.txt b/foo.txt
    index 5716ca5..7601807 100644
    --- a/foo.txt
    +++ b/foo.txt
    @@ -1 +1 @@
    -bar
    +baz

"git interpret-trailers" is unaware of the cut line, and assumes the
trailer block would be at the end of the whole thing.  This can easily
be seen with:

     $ GIT_EDITOR='git interpret-trailers --in-place --trailer Acked-by:me' \
       git commit --amend -v

Teach "git interpret-trailers" to notice the cut-line and ignore the
remainder of the input when looking for a place to add new trailer
block.  This makes it consistent with how "git commit -v -s" inserts a
new Signed-off-by: line.

This can be done by the same logic as the existing helper function,
wt_status_truncate_message_at_cut_line(), uses, but it wants the caller
to pass a strbuf to it.  Because the function ignore_non_trailer() used
by the command takes a <pointer, length> pair, not a strbuf, steal the
logic from wt_status_truncate_message_at_cut_line() to create a new
wt_status_locate_end() helper function that takes <pointer, length>
pair, and make ignore_non_trailer() call it to help "interpret-trailers".

Since there is only one caller of wt_status_truncate_message_at_cut_line()
in cmd_commit(), rewrite it to call wt_status_locate_end() helper instead
and remove the old helper that no longer has any caller.

Signed-off-by: Brian Malehorn <bmalehorn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>