Search backend: add structured diff to CommitMatch result

Created by: camdencheek

This just constructs CommitMatch with an additional field that is the structured form of the diff rather than the textual form. The text form and structural form can be converted into each other without any loss of information, so in time, we should be able to remove the textual diff from the result type.

I want this available for file:contains() on diff searches, which is why I'm spending a little time on this now.

Test plan

Updated existing tests. Things that are worth adding tests for were added in previous PRs.