parse_date: fix signedness in timezone calculation

parse_date: fix signedness in timezone calculation

When no timezone is specified, we deduce the offset by
subtracting the result of mktime from our calculated
timestamp.

However, our timestamp is stored as an unsigned integer,
meaning we perform the subtraction as unsigned. For a
negative offset, this means we wrap to a very high number,
and our numeric timezone is in the millions of hours. You
can see this bug by doing:

   $ TZ=EST \
     GIT_AUTHOR_DATE='2010-06-01 10:00' \
     git commit -a -m foo
   $ git cat-file -p HEAD | grep author
   author Jeff King <peff@peff.net> 1275404416 +119304128

Instead, we should perform this subtraction as a time_t, the
same type that mktime returns.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>