checkout: implement "-" abbreviation, add docs and tests

Have '-' mean the same as '@{-1}', i.e., the last branch we were on.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-checkout.c

@@ -679,6 +679,9 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		arg = argv[0];
 		has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");
 
+		if (!strcmp(arg, "-"))
+			arg = "@{-1}";
+
 		if (get_sha1(arg, rev)) {
 			if (has_dash_dash)          /* case (1) */
 				die("invalid reference: %s", arg);