patch: refactor content diffing part in separate fn so extensions can wrap Right now extdiff uses it's own logic using archival to diff two versions of file using external diff tools. This makes the extdiff functionality non-extensible. This series is an attempt to refactor core patch and diff functionality so that extdiff can wrap and reuse it. This will help us in using external diffing tools at more places and not just extdiff command only then. Differential Revision: https://phab.mercurial-scm.org/D8685