sshpeer: extract pipe cleanup logic to own function
So it can be used outside of instantiated classes. This is
needed to support pipe creation before __init__ is called.
Differential Revision: https://phab.mercurial-scm.org/D2029
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', [])