Allow no-merges policy

As a repository admin, I want to ensure that a branch is not only fast-forward only but also does not contain merges, which enforces committers to this branch to rebase their work properly.

This helps to keep the commit history sane and reliable to work with using external automation tools. Otherwise, the possibility exists that existing changes are missed out by i.e. reporting tools that evaluate changes between two commits.

Additional acceptance criteria:

• The branch name(s) to which this applies can be specified using standard regular expressions

This request shall enable the same functionality that is available in gitolite:

• http://gitolite.com/gitolite/g2/aac.html#mergecheck