Include "number of commits" as possible variable for merge commit templates

Mmerge commit message templates are a useful thing to configure in projects. The number of commits part of the merge would be a useful addition for the template variables. Available variables are here: https://docs.gitlab.com/ee/user/project/merge_requests/commit_templates.html#supported-variables-in-commit-templates

Number of commmits is not one of these variables.