Fix PR create regression around title and body when there is only one commit

[williammartin]

Description

Fixes #8706

Prior to #8423, when running gh pr create on a branch with a single commit, the title and body of the PR would be prefilled as if using --fill-first, using the commit summary for the title and the description for the body:

cli/pkg/cmd/pr/create/create.go

Lines 411 to 423 in dd25f88

	commits, err := gitClient.Commits(context.Background(), baseRef, headRef)
	if err != nil {
	return err
	}
	if len(commits) == 1 || useFirstCommit {
	commitIndex := len(commits) - 1
	state.Title = commits[commitIndex].Title
	body, err := gitClient.CommitBody(context.Background(), commits[commitIndex].Sha)
	if err != nil {
	return err
	}
	state.Body = body
	} else {

In #8423 a change was made such that this no longer applied for PRs with one commit. This meant that the prefilled title would be a sanitized form of the branch name, as happens when providing --fill or --fill-verbose.

This PR adds a test that validates that the commit information returned from git -c log.ShowSignature=false log --pretty=format:%H,%s,%b --cherry origin/master...feature is used in the pull request mutation when there is exactly one commit.

Reviewer Notes

I've taken the simplest approach to fixing this regression by adding the conditional back. I will follow this with another PR that introduces a new Filler abstraction to tease apart some of this intertwined logic (for example, after #8423 the commit titles will be bolded in the PR body even if there is no description, which is a change from before).

Additionally, it's also worth noting that there is a strange interaction when running gh pr create --fill with just one commit on the git branch in that the code will fall into the 1 commit branch as opposed to the expected else. However, this has always been the behaviour so I don't think we should "fix" it right now.

CC @allyjweir @guerinoni

[andyfeller]

✨ thanks for sussing this out, @williammartin

[guerinoni]

Thanks! LGTM

[andyfeller]

Just because it is late in the evening for @williammartin, I'm going to merge this and get a patch release out. Once again, thanks @williammartin for figuring this out and @werkshy for your understanding and patience. 🙇