revlog: adapt the `reading` check for `unionrepo`
We cannot just rely on the length check for the `unionrepo` as the local revlog
might be empty while the other revlog contains data. In addition, we need to
also open the second revlog for reading when needed.
$ for i in aaa zzz; do
> hg init t
> cd t
>
> echo
> echo "-- With $i"
>
> touch file
> hg add file
> hg ci -m "Add"
>
> hg cp file $i
> hg ci -m "a -> $i"
>
> hg cp $i other-file
> echo "different" >> $i
> hg ci -m "$i -> other-file"
>
> hg cp other-file somename
>
> echo "Status":
> hg st -C
> echo
> echo "Diff:"
> hg diff -g
>
> cd ..
> rm -rf t
> done
-- With aaa
Status:
A somename
other-file
Diff:
diff --git a/other-file b/somename
copy from other-file
copy to somename
-- With zzz
Status:
A somename
other-file
Diff:
diff --git a/other-file b/somename
copy from other-file
copy to somename