store: remove pointless pathjoiner parameter
We can stop pretending that we have to support anything else
than '/' for concatenating path elements anywhere.
Windows deals just fine with '/' in file paths and we already
have plenty of places which produce paths containing '/'
anyway when running on Windows.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])