Wire new date formats to --date=<format> parser.

Now we can use all internally supported date formats with

	git log --date=<format>

syntax.  Earlier, we only allowed relative/local/default.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

revision.c

@@ -1133,6 +1133,14 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, const ch
 			if (!strncmp(arg, "--date=", 7)) {
 				if (!strcmp(arg + 7, "relative"))
 					revs->date_mode = DATE_RELATIVE;
+				else if (!strcmp(arg + 7, "iso8601") ||
+					 !strcmp(arg + 7, "iso"))
+					revs->date_mode = DATE_ISO8601;
+				else if (!strcmp(arg + 7, "rfc2822") ||
+					 !strcmp(arg + 7, "rfc"))
+					revs->date_mode = DATE_RFC2822;
+				else if (!strcmp(arg + 7, "short"))
+					revs->date_mode = DATE_SHORT;
 				else if (!strcmp(arg + 7, "local"))
 					revs->date_mode = DATE_LOCAL;
 				else if (!strcmp(arg + 7, "default"))