[PATCH] Simplify date handling and make it more reliable

[PATCH] Simplify date handling and make it more reliable

This make all dates be stores as seconds since UTC epoch, with the
author's or committer's timezone as auxiliary data so that dates can be
pretty-printed in the original timezone later if anyone cares.  I left
the date parsing in rev-tree.c for backward compatibility but it can be
dropped when we change to base64 :)

commit-tree now eats RFC2822 dates as AUTHOR_DATE because that's
what you're going to want to feed it.

Yes, glibc sucks and strptime is a pile of crap. We have to parse it
ourselves.