[v12.0/forgejo] fix: make API /repos/{owner}/{repo}/compare/{basehead} work with forks (#8331)

**Backport:** https://codeberg.org/forgejo/forgejo/pulls/8326

- fix: API must use headGitRepo instead of ctx.Repo.GitRepo for comparing
- fix: make API /repos/{owner}/{repo}/compare/{basehead} work with forks
- add test coverage for both fixes and the underlying function `parseCompareInfo`
- refactor and improve part of the helpers from `tests/integration/api_helper_for_declarative_test.go`
- remove a few wrong or misleading comments

Refs forgejo/forgejo#7978

## Note on the focus of the PR

It was initially created to address a regression introduced in v12. But the tests that verify it is fixed discovered a v11.0 bug. They cannot conveniently be separated because they both relate to the same area of code that was previously not covered by any test.

## Note on v11.0 backport

It must be manually done by cherry-picking all commits up to and not including `fix: API must use headGitRepo instead of ctx.Repo.GitRepo for comparing` because it is v12 specific.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.

### Documentation

- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.

Co-authored-by: Earl Warren <contact@earl-warren.org>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8331
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>
Co-committed-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>

tests/integration/pull_commit_test.go

@@ -95,7 +95,7 @@ func TestPullCommitSignature(t *testing.T) {
 		testCtx := NewAPITestContext(t, user.Name, "pull-request-commit-header-signed", auth_model.AccessTokenScopeWriteRepository, auth_model.AccessTokenScopeWriteUser)
 		u.Path = testCtx.GitPath()
 
-		t.Run("Create repository", doAPICreateRepository(testCtx, false, git.Sha1ObjectFormat))
+		t.Run("Create repository", doAPICreateRepository(testCtx, nil, git.Sha1ObjectFormat))
 
 		t.Run("Create commit", func(t *testing.T) {
 			defer tests.PrintCurrentTest(t)()