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

utils/setup.ts

@@ -64,7 +64,6 @@ export type SetupGitParams = GitContext;
  * setup git configuration and authentication for the repository.
  * - configures git identity (user.email, user.name)
  * - sets up authentication via gitToken (minimal contents:write)
- * - for PR events, checks out the PR branch using shared helper
  *
  * gitToken is a minimal-permission token (contents + workflows) used for git operations.
  * it is assumed to be potentially exfiltratable, so it has limited scope.