revlog: drop local assignment of cache variable
The purpose of this code was to provide thread safety. With the
conversion of hgweb to use separate localrepository instances per
request/thread, we should no longer have any consumers that need to
access revlog instances from multiple threads. Remove the code.
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', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])