Improve pure Python rev-parse coverage and behavior (#2135)

Port object-resolving revspec cases inspired by gix-revision into deterministic GitPython tests, without shelling out to Git or Gix at runtime. Refactor rev_parse handling around anchors, navigation, peeling, reflog selectors, path/index lookups, describe-style names, and commit-message searches.

Document observed Git/Gix behavior differences and the GitPython choices made for user-facing compatibility.

Co-authored-by: Sebastian Thiel <sebastian.thiel@icloud.com>

Author: codex