revert: rearrange the date help

revert: remove some redundancy in basic description

revert: drop requirement to use -r to revert with two parents This reduces documentation confusion between the need to: a) hg revert -a -r . (drop all changes from a merge) b) hg up -C . (drop the second parent entirely) Currently revert is one of two commands (the other being tag) that still complains about uncommitted merges, dating from its former use of a generic defaultrev function that aborted.

revert: simplify usage note This points people looking for other commands to the right place with a minimum of verbiage.

revert: rewrite help summary New users have a tendency to mistake 'revert' as the command to use to check out old revisions. They also occasionally mistake revert for a generalized undo (compare rollback). This version intentionally aims to avoid mentioning 'earlier' and thus intentionally no longer alludes to the (secondary) -r behavior (which in fact is not actually limited to 'earlier'). Instead, we mention checkout state, to convey that we can restore things to the way they were when checked out.

extensions: make disabled()/disabledext() load prebuilt index if available __index__ contains a dict of {name: desc} of all extensions.