tests: adjust merge tool config in test-largefiles-update.t
Previously, the `ui.merge=internal:fail` configuration was shadowed by the the
`HGMERGE=internal:merge` environment variable. We would like to remove this
usage of `HGMERGE` variable because it is error-prone (eg: this very test). We
start by cleaning up test independently to avoid large churn when the change
happens.
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors