Introduction of the GitSetup

testhelper.MustRunCommand includes in it two separate flows and needs to be divided into two. One should exec git operations only and rely on the configured git executable. The other one should be used to run any other executable like du, touch, etc. This change introduces GitSetup struct that uses config.Cfg as a base to run git commands. It could be configured dynamically to use commit author, repository, etc. This change is a just a demonstration of its usage and implementation will be extended with upcoming MRs.
The goal is to replace usage of testhelper.MustRunCommand to run git with a new GitSetup and use MustRunCommand for all other executables which do not require additional configuration.

Part of: #2699 (closed)