filelog: add the ability to report the user facing name
This will be used by lfs, but is probably generally useful. There are various
bits of code that reverse engineer this from the index or data file names, but
it seems better to just store it. Especially if there's experimenting with
backing storage other than revlog.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}