.arcconfig

transaction: allow to backup file that already have an offset

This will be useful in the next changeset to deal with rolling back the split of
inline revlog on transaction failure.

This involve deeper change to the transaction logic as we need to make sure we
restore the backup -before- the truncation step. Otherwise, the truncation
would act on the wrong file and be overwritten by the backup restoration later.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}