[Feature] Expose name of deleted PR branches

[ppmathis]

• Gitea version (or commit ref): 1.11.0
• Git version: 2.24.1
• Operating system: Linux (Alpine)
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes: https://try.gitea.io/ppmathis/test/pulls/1
  • No
  • Not relevant

Description

When the head branch of a pull request in either state gets deleted, the pull request no longer exposes the original branch name. This is directly related to #9158, which requested tracking the ref to the tip of the deleted branch and a working commit/changes view.

I would like to suggest to also expose the name of the branch which got deleted through both UI and API instead of just showing from deleted into <target branch>. The name of the branch can usually be accessed as head.ref in the API, however the whole head property is null as soon as the branch got deleted.

Screenshots

PR with correct branch name before branch deletion

PR with missing branch name after branch deletion

[lunny]

Because that branch maybe from another repository which deleted.

[lafriks]

We should probably display branch name even if it is deleted just without link and marked as deleted

[ppmathis]

@lafriks That's the kind of behavior I'd expect - even when the branch or repository got deleted, the PR should still expose the original branch name through UI and API.

This also follows the implementation of all other major Git platforms including GitHub and is useful for stateless bots which can determine if a PR has been previously opened by checking for a predictable branch name.