fix timezones in darcs-fast-export

fix timezones in darcs-fast-export

Previously, the export was mangling timezones.  There were conversion errors when the darcs TZ
did not match the conversion TZ.  Also, the conversion timezone was always reported (this is
bad since two conversions may now differ).

This patch fixes both problems on my system, but it has not been extensively tested.
Can 'local_date' be used reliably?  What about my TZ manipulations?
I am no expert in darcs or python.  Developed with darcs 2.3.1 and Python 2.6.4.

To check for errors, I compared a darcs repo against a "git darcs fetch" into a new repo.
The following two commands were helpful.
# darcs changes | grep for dates
# git log --pretty=format:%ad REF | cat

Example -- EDT=UTC-4 and EST=UTC-5; I am processing in EDT.
darcs date:
"Fri Mar 31 10:48:00 EST 2006"
Before patch, git reported:
"Fri Mar 31 12:48:00 2006 -0400" (should be 11:48 -0400 or 10:48 -0500)
After patch, git reported:
"Fri Mar 31 10:48:00 2006 -0500"