use commit time as mtime for file archives.
now output from two run of "hg archive -t zip" is same.
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
pulling from ../source
abort: pretxncommit hook exited with status 1
transaction abort!
rollback completed
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
(run 'hg update' to get a working copy)
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 2 changesets, 2 total revisions