Code review comment for lp:~abentley/launchpad/no-review-diff

= Summary =
Review diffs are supplanted by preview diffs, so stop generating them.

== Proposed fix ==
This patch changes MergeProposalCreatedJob to stop generating review diffs, while still generating preview diffs.

== Pre-implementation notes ==
Pre-implementation was with thumper.

== Implementation details ==
The APIs related to review and preview diffs are somewhat different, so
some changes were necessary, such as using unicode rather than string revision ids. Also, BranchMergeProposal.preview_diff is not normally writable, so the security proxy must be removed for some tests.

== Tests ==
bin/test -vt branch-merge -t branchmerge -t test_mpcreationjobs

== Demo and Q/A ==
Not applicable; no user-visible changes.

= Launchpad lint =

Checking for conflicts. and issues in doctests and templates.
Running jslint, xmllint, pyflakes, and pylint.
Using normal rules.

Linting changed files:
  lib/lp/code/model/tests/test_branchmergeproposals.py
  lib/lp/code/browser/tests/test_branchmergeproposal.py
  lib/lp/code/scripts/tests/test_mp_creationjob.py
  lib/lp/code/stories/branches/xx-branch-merge-proposals.txt
  lib/lp/code/model/branchmergeproposaljob.py
  lib/lp/code/doc/branch-merge-proposal-notifications.txt

== Pylint notices ==

lib/lp/code/model/branchmergeproposaljob.py
    21: [F0401] Unable to import 'lazr.delegates' (No module named delegates)
    22: [F0401] Unable to import 'lazr.enum' (No module named enum)