Add commits_count variable to commit message templates

Merge and squash commit templates can list every commit message with %{all_commits}, but there is no way to include just the number of commits, which is a common thing to want in a merge commit subject (for example Merge branch 'x' (3 commits)).

This MR adds %{commits_count}, backed by MergeRequest#commits_count. It is also allowed in new merge request description templates (ALLOWED_NEW_MR_PLACEHOLDERS), where the count comes from the compared commits, the same source %{all_commits} uses there.

Documentation lists the new variable with a version note.

Resolves #383062

Differences

Before: A template such as %{title} (%{commits_count} commits) leaves the variable unexpanded.

After: The same template renders as Bugfix (3 commits). In a new MR description template the count reflects the commits being compared at creation time.

How to set up and Validate locally:

1. In a project, go to Settings > Merge requests > Merge commit message template and set it to %{title} - %{commits_count} commit(s).
2. Merge a merge request with more than one commit and check the merge commit subject.
3. Specs: bundle exec rspec spec/lib/gitlab/merge_requests/message_generator_spec.rb