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>

builtin/commit.c

@@ -1735,7 +1735,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 
 	if (verbose || /* Truncate the message just before the diff, if any. */
 	    cleanup_mode == CLEANUP_SCISSORS)
-		wt_status_truncate_message_at_cut_line(&sb);
+		strbuf_setlen(&sb, wt_status_locate_end(sb.buf, sb.len));
 
 	if (cleanup_mode != CLEANUP_NONE)
 		strbuf_stripspace(&sb, cleanup_mode == CLEANUP_ALL);

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;
 }

t/t7513-interpret-trailers.sh

@@ -1258,4 +1258,21 @@ test_expect_success 'with no command and no key' '
 	test_cmp expected actual
 '
 
+test_expect_success 'with cut line' '
+	cat >expected <<-\EOF &&
+		my subject
+
+		review: Brian
+		sign: A U Thor <author@example.com>
+		# ------------------------ >8 ------------------------
+		ignore this
+	EOF
+	git interpret-trailers --trailer review:Brian >actual <<-\EOF &&
+		my subject
+		# ------------------------ >8 ------------------------
+		ignore this
+	EOF
+	test_cmp expected actual
+'
+
 test_done

wt-status.c

@@ -896,17 +896,18 @@ conclude:
 	status_printf_ln(s, GIT_COLOR_NORMAL, "%s", "");
 }
 
-void wt_status_truncate_message_at_cut_line(struct strbuf *buf)
+size_t wt_status_locate_end(const char *s, size_t len)
 {
 	const char *p;
 	struct strbuf pattern = STRBUF_INIT;
 
 	strbuf_addf(&pattern, "\n%c %s", comment_line_char, cut_line);
-	if (starts_with(buf->buf, pattern.buf + 1))
-		strbuf_setlen(buf, 0);
-	else if ((p = strstr(buf->buf, pattern.buf)))
-		strbuf_setlen(buf, p - buf->buf + 1);
+	if (starts_with(s, pattern.buf + 1))
+		len = 0;
+	else if ((p = strstr(s, pattern.buf)))
+		len = p - s + 1;
 	strbuf_release(&pattern);
+	return len;
 }
 
 void wt_status_add_cut_line(FILE *fp)

wt-status.h

@@ -112,7 +112,7 @@ struct wt_status_state {
 	unsigned char cherry_pick_head_sha1[20];
 };
 
-void wt_status_truncate_message_at_cut_line(struct strbuf *);
+size_t wt_status_locate_end(const char *s, size_t len);
 void wt_status_add_cut_line(FILE *fp);
 void wt_status_prepare(struct wt_status *s);
 void wt_status_print(struct wt_status *s);