hg-core: silence dead-code warning by adding RevlogEntry::revion() accessor Nightly `rustc` warns about the `RevlogEntry::rev` field not being used. Rather than removing it, I added an accessor since it seems useful to be able to get the entry's revision. Differential Revision: https://phab.mercurial-scm.org/D11548

rust: remove dead code Differential Revision: https://phab.mercurial-scm.org/D11549

rust-revset: support explicit `.` revision This is basically the same thing as not specifying a revision, except this is done in the revset resolution function. This allows calls like `rhg cat some-file -r .` to work without falling back. Differential Revision: https://phab.mercurial-scm.org/D11402

rust-revset: add separate match logic for shortcuts The next change will add a shortcut for the `.` revision. One day we might start matching `tip` and others, so this is an easy refactor. Differential Revision: https://phab.mercurial-scm.org/D11401

rhg: fall back if subrepos are detected We do not handle subrepos yet, the addition of the support for `-r .` will break if we don't fall back. Differential Revision: https://phab.mercurial-scm.org/D11403

rhg: fallback if `defaults` config is set for the current command Differential Revision: https://phab.mercurial-scm.org/D11381

rhg: fallback if the current command has any generic hook defined We do not handle hooks yet. Differential Revision: https://phab.mercurial-scm.org/D11380

rhg-cat: fallback in presence of a fileset These are unsupported so far. Differential Revision: https://phab.mercurial-scm.org/D11379

rhg-cat: fallback when detecting `.` or `..` path segments We do not normalize paths correctly yet, so exclude the shortcuts. Differential Revision: https://phab.mercurial-scm.org/D11378

rhg: add support for calling `rhg cat` without a revision Turns out the necessary pieces were there already. Like the Python implementation, we default to the first parent of the dirstate. Differential Revision: https://phab.mercurial-scm.org/D11377