Make repo_path available to template strings

It can be convenient to include the absolute path to the Git repository
in the email subject or body. Users can override email subject or body by
overriding the template strings. This patch allows the user to use
%(repo_path)s in the templates.
Use repo_path from GenericEnvironment._compute_repo_shortname().
Merge branch 'repo-path'

Make the full repository path available to template strings.