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>

commit.c

@@ -11,6 +11,7 @@
 #include "commit-slab.h"
 #include "prio-queue.h"
 #include "sha1-lookup.h"
+#include "wt-status.h"
 
 static struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len, const char **);
 
@@ -1648,10 +1649,9 @@ const char *find_commit_header(const char *msg, const char *key, size_t *out_len
 /*
  * Inspect the given string and determine the true "end" of the log message, in
  * order to find where to put a new Signed-off-by: line.  Ignored are
- * trailing comment lines and blank lines, and also the traditional
- * "Conflicts:" block that is not commented out, so that we can use
- * "git commit -s --amend" on an existing commit that forgot to remove
- * it.
+ * trailing comment lines and blank lines.  To support "git commit -s
+ * --amend" on an existing commit, we also ignore "Conflicts:".  To
+ * support "git commit -v", we truncate at cut lines.
  *
  * Returns the number of bytes from the tail to ignore, to be fed as
  * the second parameter to append_signoff().
@@ -1661,8 +1661,9 @@ int ignore_non_trailer(const char *buf, size_t len)
 	int boc = 0;
 	int bol = 0;
 	int in_old_conflicts_block = 0;
+	size_t cutoff = wt_status_locate_end(buf, len);
 
-	while (bol < len) {
+	while (bol < cutoff) {
 		const char *next_line = memchr(buf + bol, '\n', len - bol);
 
 		if (!next_line)
@@ -1688,5 +1689,5 @@ int ignore_non_trailer(const char *buf, size_t len)
 		}
 		bol = next_line - buf;
 	}
-	return boc ? len - boc : 0;
+	return boc ? len - boc : len - cutoff;
 }