commit -v: strip diffs and submodule shortlogs from the commit message

When using the '-v' option of "git commit" the diff added to the commit
message temporarily for editing is stripped off after the user exited the
editor by searching for "\ndiff --git " and truncating the commmit message
there if it is found.

But this approach has two problems:

- when the commit message itself contains a line starting with
  "diff --git" it will be truncated there prematurely; and

- when the "diff.submodule" setting is set to "log", the diff may
  start with "Submodule <hash1>..<hash2>", which will be left in
  the commit message while it shouldn't.

Fix that by introducing a special scissor separator line starting with the
comment character ('#' or the core.commentChar config if set) followed by
two lines describing what it is for. The scissor line - which will not be
translated - is used to reliably detect the start of the diff so it can be
chopped off from the commit message, no matter what the user enters there.

Turn a known test failure fixed by this change into a successful test;
also add one for a diff starting with a submodule log and another one for
proper handling of the comment char.

Reported-by: Ari Pollak <ari@debian.org>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -1505,7 +1505,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	struct strbuf sb = STRBUF_INIT;
 	struct strbuf author_ident = STRBUF_INIT;
 	const char *index_file, *reflog_msg;
-	char *nl, *p;
+	char *nl;
 	unsigned char sha1[20];
 	struct ref_lock *ref_lock;
 	struct commit_list *parents = NULL, **pptr = &parents;
@@ -1601,11 +1601,8 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	}
 
 	/* Truncate the message just before the diff, if any. */
-	if (verbose) {
-		p = strstr(sb.buf, "\ndiff --git ");
-		if (p != NULL)
-			strbuf_setlen(&sb, p - sb.buf + 1);
-	}
+	if (verbose)
+		wt_status_truncate_message_at_cut_line(&sb);
 
 	if (cleanup_mode != CLEANUP_NONE)
 		stripspace(&sb, cleanup_mode == CLEANUP_ALL);