localrepo: make it possible to pass multiple path elements to join and wjoin
This makes join and wjoin behave in the same way as os.path.join. That is, it
makes it possible to pass more than one path element to them.
Note that the first path element is still required, as it was before this patch,
and as is the case for os.path.join.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..