merge: add merge action 'p' to record path conflicts during update
Add a new merge action to record path conflicts. A status message is
printed, and the path conflict is added to the merge state.
Differential Revision: https://phab.mercurial-scm.org/D776
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', [])