revlog: more efficient implementation for issnapshot
We avoid multiple method calls and tuple creation, this provides a significant
speedup in some case:
example affected manifest write
before: 0.815520s
after: 0.487767s (-40%)
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}