gate multi commit merge behind off by default flag

[jsvd]

Since it's easy to forget to squash a pull request, this change will reject a "merge" command by default if the PR has multiple commits. A new --allow-multiple-commits flag can be added to override this check.

Example error message:

@jarvis merge https://github.com/jsvd/test-repo/pull/20
:sadpanda: An error occurred in merge: RuntimeError - Pull request has more than 1 commit. Please consider squashing, or to override this check use: "merge --allow-multiple-commits pr_url target_branch .."

[jsvd]

closing for now until further process around merging multiple commit PRs is revised Some concerns raised are:

• the flag may be abused to still allow trailing commits with low quality comments
• identifying and applying a reasonable auto squash merge strategy would simplify the merging + backporting, even in the cases of multiple authors.