Allow 'squash and merge' for single-commit PRs

[damithc]

May be we can use the 'squash and merge' option for single commit PRs. That way, the person merging can change the commit message to match the merge commit format while giving a cleaner commit history (i.e. no unnecessary merge commits).

[wkurniawan07]

For single commits, "squash-and-merge" and "rebase-and-merge" practically has no difference other than the former allows the commit message to be set.