Add variable to support adding all commits to squash template

Problem to solve

When editing the squash commit message, the user should be able to have a review of the commit messages for the commits that will be squashed and be able edit them.

When squashing commits prior to merging, it can be helpful to include the details of all the commits that will be squashed so that users can understand the complete context of the change.

Proposal

Create a new variable %{all_commit_messages} for squash commit templates that adds the commits on squash.

Additional Details

The format of the included commits should be:

* Commit title 1

Commit body 1

* Commit title 2

Commit body 2