This fixes #435 (and also #427).

What/why

As noted in the issue, git spr update can overwrite changes made to the PR title/description. To fix this, we can have a dedicated block that looks like this, which the code can search for.

<!-- SPR data start: please do NOT edit this section -->
  
<!-- SPR data end -->

This way, any edits outside of this block are ignored. Since the block uses comments, it visually looks the same as before. And finally, if no block is present (e.g. on first commit or deleted by a user), it appends to the end.

Test plan

I added new test cases. In addition, I ran a manual test which you can see in https://github.com/rohan-mehta/spr-test-repo. The process was:

1. Create 3 commits, commit_1, commit_2, commit_3, run git spr update, ensure that the spr stack description shows up.
2. Edit the PR for commit_1 (add stuff above the block) and commit_2 (add stuff above and below the block), then re-run git spr update and ensure that nothing is overwritten.
3. Amend commit_2, re-run git spr update and ensure that nothing is overwritten.
4. Create commit_4 at the top of the stack, re-run git spr update and ensure that (a) a new PR is created (b) all previous PRs are updated to show the new commit (c) edits aren't overwritten