largefile: use parentchange during mergerecordupdates
This match what core is doing around recordupdates.
Differential Revision: https://phab.mercurial-scm.org/D11105
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL