Use commit message as PR title if PR consists only of one commit

[alexmt]

If useUniqueBranchNames is set to false the kargo-render uses generic <branch name> <-- latest batched changes message even if PR has one commit:

kargo-render/prs.go

Line 21 in 261e1bc

fmt.Sprintf("%s <-- latest batched changes", rc.request.TargetBranch)

However often PR has only one commit. In our organization, it is 99% of cases. It is proposed to use commit message as PR title if PR consists of only one commit. It would really improve the usability and save us from additional click everytime we need to know what was deployed using Argo CD.