[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_squash.go

@@ -5,7 +5,6 @@ package pull
 
 import (
 	"fmt"
-	"strings"
 
 	repo_model "forgejo.org/models/repo"
 	user_model "forgejo.org/models/user"
@@ -67,10 +66,8 @@ func doMergeStyleSquash(ctx *mergeContext, message string) error {
 
 	if setting.Repository.PullRequest.AddCoCommitterTrailers && ctx.committer.String() != sig.String() {
 		// add trailer
-		if !strings.Contains(message, fmt.Sprintf("Co-authored-by: %s", sig.String())) {
-			message += fmt.Sprintf("\nCo-authored-by: %s", sig.String())
-		}
-		message += fmt.Sprintf("\nCo-committed-by: %s\n", sig.String())
+		message = AddCommitMessageTrailer(message, "Co-authored-by", sig.String())
+		message = AddCommitMessageTrailer(message, "Co-committed-by", sig.String()) // FIXME: this one should be removed, it is not really used or widely used
 	}
 	cmdCommit := git.NewCommand(ctx, "commit").
 		AddOptionFormat("--author='%s <%s>'", sig.Name, sig.Email).