We noticed in https://gitlab.com/gitlab-org/gitaly/issues/2395 that the
backend was unnecessarily attempting to squash merge requests with one
commit. This happened because `SquashService` skips the squash if two
conditions are met:

1. The number of commits < 2
2. The `squash_commit_message` is null

If a `squash_commit_message` is passed, this indicates that the user
wants the squashed commit to have a different commit message.

By default, the frontend always sends `squash_commit_message`,
regardless of whether the squash button is available in the merge
request. With this change, the frontend will only send this parameter if
the button is shown (i.e. when the commit count is >= 2).

Alternatively, we could also change the backend to skip squashing if
only the first condition is met.

Closes https://gitlab.com/gitlab-org/gitlab/issues/198971