Fix an issue where added or removed source files could incorrectly select a DocumentEngine

Summary:
Ref T13425. The changes in D20865 could incorrectly lead to selection of a DocumentEngine that can not generate document diffs if a file was added or removed (for example, when a source file is added).

Move the engine pruning code to be shared -- we should always discard engines which can't generate a diff, even if we don't have both documents.

Test Plan: Viewed an added source file, no more document ref error arising from document engine selection.

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20866