shelve: move method for getting stat (mtime) to new shelf class
Only the mtime was needed, so I made it restricted to that in the
move.
The new `Shelf` class expects its argument to be a shelf name (not a
arbitrary filename like `shelvedfile` would accept), so only the
shelf name is now passed in.
Differential Revision: https://phab.mercurial-scm.org/D9707
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}