template: add predecessors template Add a 'predecessors' template that returns the list of all closest known predecessors for a changectx. The elements of the list are row changectx node id formatted by default as short nodes. The "closest predecessors" are the first locally known revisions encountered while, walking predecessors markers. For example: 1) If a (A, (B)) markers exists and both A and B are locally known A is a closest predecessors of B. 2) If a (A, (B)) and (B, (C)) markers exists and only A and C are known locally, A will be the closest precursors of C. This logic respect repository filtering. So hidden revision will be skipped by this logic unless --hidden is specified. Since we only display the visible predecessors, this template will not display anything in most case. It makes a good candidate for inclusion in the default log output. I added a new test-file for testing the precursors in various scenarios. This test file will also be used for the successors template. A new "obsutil" module has been added to start gathering utility function outside of the large obsolete.py module.