Code review comment for lp:~vila/bzr/per-file-merge-hook

Since spiv is on leave, I've finished his lp:~spiv/bzr/per-file-merge-hook-491711
merge proposal based on:
- the reviews,
- some discussion on IRC,
- some more cleanup.

The core modification is about caching the config data to avoid re-reading
the config files for each merged file.

* bzrlib/tests/per_merger.py:
Fix line too long and spurious spaces.

* bzrlib/plugins/news_merge/tests/test_news_merge.py:
(TestFilenameMatchesConfig): Ensure that the params get updated.

* bzrlib/plugins/news_merge/__init__.py:
(filename_matches_config): Save the relevant config variable in
the hook params.
(install_hook): Wrap the hook installation so we can reuse it for
tests.

* bzrlib/plugins/news_merge/README:
Update the instructions by pointing to the plugin help.

* bzrlib/merge.py:
(MergeHookParams): Delete spurious spaces.

* bzrlib/decorators.py:
(use_pretty_decorators): Mention that we get clearance to copy
launchpad code here (since canonical has copyrights on both code
bases).

As it's far too much to be considered simple tweaks, I'd like a review on the diff
before (if approved :) landing both.