When making comments on diffs for review requests that have multiple commits, there were a handful of bugs of varying severity. These all occurred when adding comments to subsets of the commit range, rather than the cumulative diff. - Medium bad: The link URL for comments added to commit ranges went to the regular diff page without including any commit info in the query string. We therefore wouldn't load the correct view. - Bad: When looking at commit ranges, we'd add comment flags to the diff view for all comments where the tip filediff matched. This would end up showing comment flags even if the base commit didn't match. - Very bad: The diff fragment shown for comments on commit ranges wasn't taking the base commit into account, instead always showing the diff between the base of the entire change and the selected tip commit. In most cases this wouldn't matter but if there are repeated changes to the same file across many commits, the shown diff fragment would be completely wrong. These bugs all more or less come down to the same underlying problem: not plumbing through the right commit information. We thankfully are correctly storing the "base filediff" in the comment `extra_data`, which is the ID of the filediff for the given file in the selected base commit of the range. We therefore need to plumb that through so that we're always getting the correct things from `get_diff_files` and other diffutils methods. In order to make this testable, I've pulled some commit-specific test infrastructure out of test_diffutils.py and made it available from our main testcase file. Testing Done: - Ran unit tests. - Posted a review request with a number of commits that all changed the same file. Added comments across various ranges of those commits. Saw the following: - In the review box, the link for comments always went to the correct commit range for the comment. - In the review box, the diff fragment for the comment was the same as what was selected when making the comment. - When viewing commit ranges in the diff viewer, I only saw comments that matched the currently visible commit range.