Add an option to perform a full clone

By default, the plugin performs clones with the `--rev` option. According to the Mercurial man page, this:

    To pull only a subset of changesets, specify one or more
    revisions identifiers with -r/--rev or branches with
    -b/--branch. The resulting clone will contain only the
    specified changesets and their ancestors.

On very large repositories, the ancestor calculation can put a large load on the Mercurial server, resulting in slow clone times.  Doing a full clone is actually much quicker, even though the clone is larger.  For example, on our largest repository (500k+ files, 40k+ changesets) this reduces initial clone times from 3.5 hours to 25 minutes!

This adds an option for doing a full clone.  To stay backwards compatible, full clones are off by default.

**NOTE:** Since version 1.50 hasn't been released yet, I didn't add a new deprecated constructor.  If you'd rather I add this, just let me know.