feat(third_party/git/date): add "dottime" format

Adds dottime (as defined on https://dotti.me) as a timestamp format.

This format is designed to simplify working with timestamps across
many different timezones by keeping the timestamp format itself in
UTC (and indicating this with a dot character), but appending the
local offset.

This is implemented as a new format because the timestamp needs to be
rendered both as UTC and including the offset, an implementation using
a strftime formatting string is not sufficient.

third_party/git/builtin/blame.c

@@ -989,6 +989,9 @@ parse_done:
 	case DATE_STRFTIME:
 		blame_date_width = strlen(show_date(0, 0, &blame_date_mode)) + 1; /* add the null */
 		break;
+	case DATE_DOTTIME:
+		blame_date_width = sizeof("2006-10-19T15·00-0700");
+		break;
 	}
 	blame_date_width -= 1; /* strip the null */