allow name of main development branch to be configurable

[brobecke]

Some repositories do not use the name "master" for their main development branch; for instance, the "newlib-cygwin" repository uses the name "main" instead. Since then, the subject of the commit email now include the name "main" unlike what is done for the master branch, where the name of the branch is omitted, on the assumption that "master" is the name of the development branch.

The purpose of this issue is to enhance the git-hooks to allow the name of the main development branch to be configurable.