rust: Add a Filelog struct that wraps Revlog
Some filelog-specific logic is moved from code `rhg cat` into this struct
where it can better be reused.
Additionally, a missing end delimiter for metadata causes an error
to be returned instead of being silently ignored.
Differential Revision: https://phab.mercurial-scm.org/D11408
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}