Add local_reference (Merge Request ID) to commit template

What does this MR do and why?

This PR adds a %{local_reference} Commit message templates variable that resolves to a simple merge request ID like !123. Currently, %{reference} resolves to the full MR reference like path/to/group/project!123.

I'd like to reference IDs in the commit name like GitHub does by default with squash merges (commit message (#PR-ID)).

Solves #199823

TODO

• Add tests to https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/lib/gitlab/merge_requests/message_generator_spec.rb
• [Help wanted] Settle on a name for this variable. local_reference may not be ideal. Here's a list of possible options I found:
  • local_reference (currently implemented)
  • short_reference
  • id
  • mr_id
• [Help wanted] Update the documentation to reflect the right introduction version.

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