[gitea] Always use an empty line to separate the commit message and trailer (#8041)

This is a port of a gitea PR: https://github.com/go-gitea/gitea/pull/34512.

I have added some copy-editing commits on top for cleanliness.
I haven't tested the changes manually and only relied on the existing automated test.

## Checklist
### Tests

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

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [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.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Co-authored-by: Jim Lin <jim@andestech.com>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8041
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Antonin Delpeuch <antonin@delpeuch.eu>
Co-committed-by: Antonin Delpeuch <antonin@delpeuch.eu>

services/pull/merge_test.go

@@ -65,3 +65,28 @@ func Test_expandDefaultMergeMessage(t *testing.T) {
 		})
 	}
 }
+
+func TestAddCommitMessageTailer(t *testing.T) {
+	// add tailer for empty message
+	assert.Equal(t, "\n\nTest-tailer: TestValue", AddCommitMessageTrailer("", "Test-tailer", "TestValue"))
+
+	// add tailer for message without newlines
+	assert.Equal(t, "title\n\nTest-tailer: TestValue", AddCommitMessageTrailer("title", "Test-tailer", "TestValue"))
+	assert.Equal(t, "title\n\nNot tailer: xxx\n\nTest-tailer: TestValue", AddCommitMessageTrailer("title\n\nNot tailer: xxx", "Test-tailer", "TestValue"))
+	assert.Equal(t, "title\n\nNotTailer: xxx\n\nTest-tailer: TestValue", AddCommitMessageTrailer("title\n\nNotTailer: xxx", "Test-tailer", "TestValue"))
+	assert.Equal(t, "title\n\nnot-tailer: xxx\n\nTest-tailer: TestValue", AddCommitMessageTrailer("title\n\nnot-tailer: xxx", "Test-tailer", "TestValue"))
+
+	// add tailer for message with one EOL
+	assert.Equal(t, "title\n\nTest-tailer: TestValue", AddCommitMessageTrailer("title\n", "Test-tailer", "TestValue"))
+
+	// add tailer for message with two EOLs
+	assert.Equal(t, "title\n\nTest-tailer: TestValue", AddCommitMessageTrailer("title\n\n", "Test-tailer", "TestValue"))
+
+	// add tailer for message with existing tailer (won't duplicate)
+	assert.Equal(t, "title\n\nTest-tailer: TestValue", AddCommitMessageTrailer("title\n\nTest-tailer: TestValue", "Test-tailer", "TestValue"))
+	assert.Equal(t, "title\n\nTest-tailer: TestValue\n", AddCommitMessageTrailer("title\n\nTest-tailer: TestValue\n", "Test-tailer", "TestValue"))
+
+	// add tailer for message with existing tailer and different value (will append)
+	assert.Equal(t, "title\n\nTest-tailer: v1\nTest-tailer: v2", AddCommitMessageTrailer("title\n\nTest-tailer: v1", "Test-tailer", "v2"))
+	assert.Equal(t, "title\n\nTest-tailer: v1\nTest-tailer: v2", AddCommitMessageTrailer("title\n\nTest-tailer: v1\n", "Test-tailer", "v2"))
+}