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/api_helper_for_declarative_test.go

@@ -48,20 +48,22 @@ func (ctx APITestContext) GitPath() string {
 	return fmt.Sprintf("%s/%s.git", ctx.Username, ctx.Reponame)
 }
 
-func doAPICreateRepository(ctx APITestContext, empty bool, objectFormat git.ObjectFormat, callback ...func(*testing.T, api.Repository)) func(*testing.T) {
+func doAPICreateRepository(ctx APITestContext, opts *api.CreateRepoOption, objectFormat git.ObjectFormat, callback ...func(*testing.T, api.Repository)) func(*testing.T) {
 	return func(t *testing.T) {
-		createRepoOption := &api.CreateRepoOption{
-			AutoInit:         !empty,
-			Description:      "Temporary repo",
-			Name:             ctx.Reponame,
-			Private:          true,
-			Template:         true,
-			Gitignores:       "",
-			License:          "WTFPL",
-			Readme:           "Default",
-			ObjectFormatName: objectFormat.Name(),
+		if opts == nil {
+			opts = &api.CreateRepoOption{
+				AutoInit:    true,
+				Description: "Temporary repo",
+				Name:        ctx.Reponame,
+				Private:     true,
+				Template:    true,
+				Gitignores:  "",
+				License:     "WTFPL",
+				Readme:      "Default",
+			}
 		}
-		req := NewRequestWithJSON(t, "POST", "/api/v1/user/repos", createRepoOption).
+		opts.ObjectFormatName = objectFormat.Name()
+		req := NewRequestWithJSON(t, "POST", "/api/v1/user/repos", opts).
 			AddTokenAuth(ctx.Token)
 		if ctx.ExpectedCode != 0 {
 			ctx.Session.MakeRequest(t, req, ctx.ExpectedCode)
@@ -237,8 +239,8 @@ func doAPICreatePullRequest(ctx APITestContext, owner, repo, baseBranch, headBra
 	}
 }
 
-func doAPIGetPullRequest(ctx APITestContext, owner, repo string, index int64) func(*testing.T) (api.PullRequest, error) {
-	return func(t *testing.T) (api.PullRequest, error) {
+func doAPIGetPullRequest(ctx APITestContext, owner, repo string, index int64) func(*testing.T) api.PullRequest {
+	return func(t *testing.T) api.PullRequest {
 		req := NewRequest(t, http.MethodGet, fmt.Sprintf("/api/v1/repos/%s/%s/pulls/%d", owner, repo, index)).
 			AddTokenAuth(ctx.Token)
 
@@ -248,10 +250,9 @@ func doAPIGetPullRequest(ctx APITestContext, owner, repo string, index int64) fu
 		}
 		resp := ctx.Session.MakeRequest(t, req, expected)
 
-		decoder := json.NewDecoder(resp.Body)
 		pr := api.PullRequest{}
-		err := decoder.Decode(&pr)
-		return pr, err
+		DecodeJSON(t, resp, &pr)
+		return pr
 	}
 }
 
@@ -347,20 +348,40 @@ func doAPICancelAutoMergePullRequest(ctx APITestContext, owner, repo string, ind
 	}
 }
 
-func doAPIGetBranch(ctx APITestContext, branch string, callback ...func(*testing.T, api.Branch)) func(*testing.T) {
-	return func(t *testing.T) {
+func doAPIGetBranch(ctx APITestContext, branch string) func(*testing.T) api.Branch {
+	return func(t *testing.T) api.Branch {
 		req := NewRequestf(t, "GET", "/api/v1/repos/%s/%s/branches/%s", ctx.Username, ctx.Reponame, branch).
 			AddTokenAuth(ctx.Token)
+		expected := http.StatusOK
+		if ctx.ExpectedCode != 0 {
+			expected = ctx.ExpectedCode
+		}
+		resp := ctx.Session.MakeRequest(t, req, expected)
+
+		branch := api.Branch{}
+		DecodeJSON(t, resp, &branch)
+		return branch
+	}
+}
+
+func doAPICreateTag(ctx APITestContext, tag, target, message string, callback ...func(*testing.T, api.Tag)) func(*testing.T) {
+	return func(t *testing.T) {
+		req := NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/%s/tags", ctx.Username, ctx.Reponame), &api.CreateTagOption{
+			TagName: tag,
+			Message: message,
+			Target:  target,
+		}).
+			AddTokenAuth(ctx.Token)
 		if ctx.ExpectedCode != 0 {
 			ctx.Session.MakeRequest(t, req, ctx.ExpectedCode)
 			return
 		}
-		resp := ctx.Session.MakeRequest(t, req, http.StatusOK)
+		resp := ctx.Session.MakeRequest(t, req, http.StatusCreated)
 
-		var branch api.Branch
-		DecodeJSON(t, resp, &branch)
+		var tag api.Tag
+		DecodeJSON(t, resp, &tag)
 		if len(callback) > 0 {
-			callback[0](t, branch)
+			callback[0](t, tag)
 		}
 	}
 }