Extract file path information from diffs directly

This somewhat duplicated logic from Gitlab::Git::File#file_path,
but to get at file path information through those objects, via
@diff_collection#diff_files, we trigger a circular and confusing
call back to this class. Copying the logic here isn't DRY, but it
reduces the cognitive load for devs working in this code, avoids the
potential circular reference, and generally makes life easier for
reviewers.

lib/gitlab/diff/highlight_cache.rb

@@ -112,6 +112,7 @@ module Gitlab
         deprecated_cache.clear
       end
 
+      #
       def file_paths
         strong_memoize(:file_paths) do
           @diff_collection.diffs.collect(&:file_path)