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/hg-core/src/revset.rs

@@ -19,6 +19,10 b' pub fn resolve_single('
     let changelog = repo.changelog()?;
 
     match input {
+        "." => {
+            let p1 = repo.dirstate_parents()?.p1;
+            return Ok(changelog.revlog.rev_from_node(p1.into())?);
+        }
         "null" => return Ok(NULL_REVISION),
         _ => {}
     }