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
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true