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
a
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions
a not present
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
a