track-tags: introduce first bits of tags tracking during transaction This changeset introduces detection of tags changes during transaction. When this happens a 'tag_moved=1' argument is set for hooks, similar to what we do for bookmarks and phases. This code is disabled by default as there are still various performance concerns. Some require a smarter use of our existing tag caches and some other require rework around the transaction logic to skip execution when unneeded. These performance improvements have been delayed, I would like to be able to experiment and stabilize the feature behavior first. Later changesets will push the concept further and provide a way for hooks to know what are the actual changes introduced by the transaction. Similar work is needed for the other families of changes (bookmark, phase, obsolescence, etc). Upgrade of the transaction logic will likely be performed at the same time. The current code can report some false positive when .hgtags file changes but resulting tags are unchanged. This will be fixed in the next changeset. For testing, we simply globally enable a hook in the tag test as all the possible tag update cases should exist there. A couple of them show the false positive mentioned above. See in code documentation for more details.