fix: make API /repos/{owner}/{repo}/compare/{basehead} work with forks (#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.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8326
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

tests/integration/ssh_key_test.go

@@ -55,7 +55,13 @@ func testPushDeployKeyOnEmptyRepo(t *testing.T, u *url.URL) {
 		keyname := fmt.Sprintf("%s-push", ctx.Reponame)
 		u.Path = ctx.GitPath()
 
-		t.Run("CreateEmptyRepository", doAPICreateRepository(ctx, true, objectFormat))
+		opts := &api.CreateRepoOption{
+			Description: "Temporary repo",
+			Name:        ctx.Reponame,
+			Private:     true,
+			Template:    true,
+		}
+		t.Run("CreateEmptyRepository", doAPICreateRepository(ctx, opts, objectFormat))
 
 		t.Run("CheckIsEmpty", doCheckRepositoryEmptyStatus(ctx, true))
 
@@ -105,8 +111,8 @@ func testKeyOnlyOneType(t *testing.T, u *url.URL) {
 	failCtx := ctx
 	failCtx.ExpectedCode = http.StatusUnprocessableEntity
 
-	t.Run("CreateRepository", doAPICreateRepository(ctx, false, git.Sha1ObjectFormat))           // FIXME: use forEachObjectFormat
-	t.Run("CreateOtherRepository", doAPICreateRepository(otherCtx, false, git.Sha1ObjectFormat)) // FIXME: use forEachObjectFormat
+	t.Run("CreateRepository", doAPICreateRepository(ctx, nil, git.Sha1ObjectFormat))           // FIXME: use forEachObjectFormat
+	t.Run("CreateOtherRepository", doAPICreateRepository(otherCtx, nil, git.Sha1ObjectFormat)) // FIXME: use forEachObjectFormat
 
 	withKeyFile(t, keyname, func(keyFile string) {
 		var userKeyPublicKeyID int64
@@ -180,7 +186,7 @@ func testKeyOnlyOneType(t *testing.T, u *url.URL) {
 
 			t.Run("DeleteOtherRepository", doAPIDeleteRepository(otherCtxWithDeleteRepo))
 
-			t.Run("RecreateRepository", doAPICreateRepository(ctxWithDeleteRepo, false, git.Sha1ObjectFormat)) // FIXME: use forEachObjectFormat
+			t.Run("RecreateRepository", doAPICreateRepository(ctxWithDeleteRepo, nil, git.Sha1ObjectFormat)) // FIXME: use forEachObjectFormat
 
 			t.Run("CreateUserKey", doAPICreateUserKey(ctx, keyname, keyFile, func(t *testing.T, publicKey api.PublicKey) {
 				userKeyPublicKeyID = publicKey.ID