Set GIT_COMMITTER_DATE and GIT_AUTHOR_DATE

Set GIT_COMMITTER_DATE and GIT_AUTHOR_DATE

This avoids most date dependencies: unless other measures are taken,
the commits and tags are identical iff they are constructed
identically.  This makes the test suite more deterministic.

In drs-push-rejects test, increment them explicitly, so as to make all
of mktag's tags distinct objects.

Otherwise it can generate identical tags (now, always; previously,
only sometimes on fast computers), causing spurious variations in
behaviour (eg, things not being rejected because they do not represent
any change).