commit/diff search: track matched file diffs for better merge

Created by: camdencheek

This modifies our matching code to track which file diffs were matched so we can merge them correctly. This allows us to run something like file:a b and only return diffs that contain file diffs that both match file:a and match content b. Previously, this would be interpreted as "(diff contains a file diff that matches file:a) or (diff contains a file diff with content modification matching b)".

Note that I'd been previously using the word "hunks" to describe this issue, but we more accurately operate on file diffs, not hunks. For reference: diff > file diff (also referred to as "patch") > hunk > line.

Fixes #25755 (closed)