notes: add shorthand --ref to override GIT_NOTES_REF

Adds a shorthand option that overrides the GIT_NOTES_REF variable, and
hence determines the notes tree that will be manipulated.  It also
DWIMs a refs/notes/ prefix.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-notes.txt

@@ -116,6 +116,11 @@ OPTIONS
 	Like '-C', but with '-c' the editor is invoked, so that
 	the user can further edit the note message.
 
+--ref <ref>::
+	Manipulate the notes tree in <ref>.  This overrides both
+	GIT_NOTES_REF and the "core.notesRef" configuration.  The ref
+	is taken to be in `refs/notes/` if it is not qualified.
+
 Author
 ------
 Written by Johannes Schindelin <johannes.schindelin@gmx.de> and

builtin-notes.c

@@ -447,6 +447,7 @@ int cmd_notes(int argc, const char **argv, const char *prefix)
 	int given_object = 0, i = 1, retval = 0;
 	struct msg_arg msg = { 0, 0, STRBUF_INIT };
 	const char *rewrite_cmd = NULL;
+	const char *override_notes_ref = NULL;
 	struct option options[] = {
 		OPT_GROUP("Notes options"),
 		OPT_CALLBACK('m', "message", &msg, "MSG",
@@ -459,6 +460,8 @@ int cmd_notes(int argc, const char **argv, const char *prefix)
 			   "reuse specified note object", parse_reuse_arg),
 		OPT_BOOLEAN('f', "force", &force, "replace existing notes"),
 		OPT_BOOLEAN(0, "stdin", &from_stdin, "read objects from stdin"),
+		OPT_STRING(0, "ref", &override_notes_ref, "notes_ref",
+			   "use notes from <notes_ref>"),
 		OPT_STRING(0, "for-rewrite", &rewrite_cmd, "command",
 			   "load rewriting config for <command> (implies --stdin)"),
 		OPT_END()
@@ -468,6 +471,19 @@ int cmd_notes(int argc, const char **argv, const char *prefix)
 
 	argc = parse_options(argc, argv, prefix, options, git_notes_usage, 0);
 
+	if (override_notes_ref) {
+		struct strbuf sb = STRBUF_INIT;
+		if (!prefixcmp(override_notes_ref, "refs/notes/"))
+			/* we're happy */;
+		else if (!prefixcmp(override_notes_ref, "notes/"))
+			strbuf_addstr(&sb, "refs/");
+		else
+			strbuf_addstr(&sb, "refs/notes/");
+		strbuf_addstr(&sb, override_notes_ref);
+		setenv("GIT_NOTES_REF", sb.buf, 1);
+		strbuf_release(&sb);
+	}
+
 	if (argc && !strcmp(argv[0], "list"))
 		list = 1;
 	else if (argc && !strcmp(argv[0], "add"))