largefile: use parentchange during mergerecordupdates
This match what core is doing around recordupdates.
Differential Revision: https://phab.mercurial-scm.org/D11105
largefile: synchronise the dirstate within a `parentchange` context
Semantically these changes are made to adjust the dirstate after a commit, so it
should be in a `parentchange` context.
Differential Revision: https://phab.mercurial-scm.org/D11103