Add 'geany_commit_utils' module to reduce code duplication

This module just holds some helper functions used in multiple scripts
like logging setup and command execution.
On GIT mirror respository update, write an agefile for CGit

CGit (the web interface for Geany's GIT mirror repositories) can be fed with an
agefile to tell it the time of the last commit.
To not clutter the repositories too much, we introduce a new sub directory _geany.
Various cleanups