Misc: switch to single export operation in "zip" target.

While exporting parts of the tree might be better in some cases, it
is awfully slow overseas, and also requires unlocking ssh key multiple
times.  Exporting the whole repo and removing directories not needed for
zip is faster here.

It is also a required step before we can switch to Mercurial.
Misc: removed unused "snapshot" target.
Misc: support for Mercurial repositories.