contrib: introduce an all-revsets.txt file This file should gather all revsets ever thought interesting by anyone. That way one can check the impact of a change when touching something revset-ish. See inline comments for details. This file have been refilled with all the entry I could automatically find from changeset descriptions. I assume we missed some not using 'revsetbenchmarks.py' output.

contrib: rename revsetbenchmarks.txt to 'base-revsets.txt' We rename the file and document its purpose. We'll be introducing another file gathering revsets useful for benchmark of the predicate themsleves in a coming changesets.

revsetbenchmarks: add main documention for the script This allow us to document the fact we can use comment in the file listing revsets.