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/cache.h

@@ -1498,7 +1498,8 @@ enum date_mode_type {
 	DATE_RFC2822,
 	DATE_STRFTIME,
 	DATE_RAW,
-	DATE_UNIX
+	DATE_UNIX,
+	DATE_DOTTIME
 };
 
 struct date_mode {