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.
https://bz.mercurial-scm.org/2493
Testing tagging with the EOL extension
$ cat >> $HGRCPATH <<EOF
> [extensions]
> eol =
>
> [eol]
> native = CRLF
> EOF
setup repository
$ hg init repo
$ cd repo
$ cat > .hgeol <<EOF
> [patterns]
> ** = native
> EOF
$ printf "first\r\nsecond\r\nthird\r\n" > a.txt
$ hg commit --addremove -m 'checkin'
adding .hgeol
adding a.txt
Tag:
$ hg tag 1.0
Rewrite .hgtags file as it would look on a new checkout:
$ hg update -q null
$ hg update -q
Touch .hgtags file again:
$ hg tag 2.0
$ cd ..