resolve: fix mark-check when a file was deleted on one side (issue6020)

Authored by spectral.

Description

resolve: fix mark-check when a file was deleted on one side (issue6020)

wvfs.open raises an error if one of the files does not exist. Ignoring the error
if it's ENOENT is done in several other places in this code, so I'm in good
company :)

Differential Revision: https://phab.mercurial-scm.org/D5243