annotate CONTRIBUTING @ 31994:b36318e6d2ef

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.

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