Rework incremental diffing (#499)

* Improve our deepening logic

* Use consistent SHA for PR-related operations in CheckoutPrTool

* compute `deepenDepth` at more appropriate time

* fix stale comment

* add comments for `alreadyOnBranch`

* ensure before sha is available

* small cleanup

* computeIncrementalDiff

* move the util

* improve algorithm

* improve algorithm further

* get rid of temp result array

* add comment

* compute incremental diff and updte instructions

* add comment

* update stale comment

* get rid of redundant rev-parse call

* improve comment

* strenghten the instructions

* make diff paths unique

external.ts

@@ -212,7 +212,7 @@ interface PullRequestSynchronizeEvent extends BasePayloadEvent {
   title: string;
   body: string | null;
   branch: string;
-  /** SHA before the push -- used to compute incremental diff via `git diff before_sha...HEAD` */
+  /** SHA before the push -- used to compute incremental range-diff between PR versions */
   before_sha: string;
 }