D5243: resolve: fix mark-check when a file was deleted on one side (issue6020)

[Yuya Nishihara]

> -                    with repo.wvfs(f) as fobj:
> -                        fdata = fobj.read()
> +                    try:
> +                        with repo.wvfs(f) as fobj:
> +                            fdata = fobj.read()
> +                    except (IOError, OSError) as inst:
> +                        if inst.errno != errno.ENOENT:
> +                            raise

'fdata' can be undefined here. Appears that the test doesn't go through this
case.

Perhaps, `wvfs.tryread()` can be used instead.

>                      if filemerge.hasconflictmarkers(fdata) and \
>                          ms[f] != mergemod.MERGE_RECORD_RESOLVED:
>                          hasconflictmarkers.append(f)