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.
$ hg init
$ echo a > a
$ hg ci -qAm 'add a'
$ hg init subrepo
$ echo 'subrepo = http://example.net/libfoo' > .hgsub
$ hg ci -qAm 'added subrepo'
$ hg up -qC 0
$ echo ax > a
$ hg ci -m 'changed a'
created new head
$ hg up -qC 1
$ cd subrepo
$ echo b > b
$ hg add b
$ cd ..
Should fail, since there are added files to subrepo:
$ hg merge
abort: uncommitted changes in subrepository 'subrepo'
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