git commit --author=$name: look $name up in existing commits

This allows "git commit --author=$name" to accept a name that is not in
the required "A U Thor <author@example.xz>" format, and use that to look
up an author name that matches from existing commits.

When using this feature, it is the user's responsibility to give a name
that uniquely matches the name s/he wants, as the logic returns the name
from the first matching commit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -75,8 +75,10 @@ OPTIONS
 	read the message from the standard input.
 
 --author=<author>::
-	Override the author name used in the commit.  Use
-	`A U Thor <author@example.com>` format.
+	Override the author name used in the commit.  You can use the
+	standard `A U Thor <author@example.com>` format.  Otherwise,
+	an existing commit that matches the given string and its author
+	name is used.
 
 -m <msg>::
 --message=<msg>::

builtin-commit.c

@@ -710,6 +710,31 @@ static int message_is_empty(struct strbuf *sb, int start)
 	return 1;
 }
 
+static const char *find_author_by_nickname(const char *name)
+{
+	struct rev_info revs;
+	struct commit *commit;
+	struct strbuf buf = STRBUF_INIT;
+	const char *av[20];
+	int ac = 0;
+
+	init_revisions(&revs, NULL);
+	strbuf_addf(&buf, "--author=%s", name);
+	av[++ac] = "--all";
+	av[++ac] = "-i";
+	av[++ac] = buf.buf;
+	av[++ac] = NULL;
+	setup_revisions(ac, av, &revs, NULL);
+	prepare_revision_walk(&revs);
+	commit = get_revision(&revs);
+	if (commit) {
+		strbuf_release(&buf);
+		format_commit_message(commit, "%an <%ae>", &buf, DATE_NORMAL);
+		return strbuf_detach(&buf, NULL);
+	}
+	die("No existing author found with '%s'", name);
+}
+
 static int parse_and_validate_options(int argc, const char *argv[],
 				      const char * const usage[],
 				      const char *prefix)
@@ -720,6 +745,9 @@ static int parse_and_validate_options(int argc, const char *argv[],
 	logfile = parse_options_fix_filename(prefix, logfile);
 	template_file = parse_options_fix_filename(prefix, template_file);
 
+	if (force_author && !strchr(force_author, '>'))
+		force_author = find_author_by_nickname(force_author);
+
 	if (logfile || message.len || use_message)
 		use_editor = 0;
 	if (edit_flag)