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.
os = <unloaded module 'os'>
os.system = <built-in function system>
os = <module 'os' from '?'>
util = <unloaded module 'util'>
util.system = <function system at 0x?>
util = <module 'mercurial.util' from '?'>
util.system = <function system at 0x?>
fred = <unloaded module 're'>
re = <unloaded module 'sys'>
fred = <unloaded module 're'>
fred.sub = <function sub at 0x?>
fred = <proxied module 're'>
re = <unloaded module 'sys'>
re.stderr = <open file '<whatever>', mode 'w' at 0x?>
re = <proxied module 'sys'>
node = <module 'mercurial.node' from '?'>