LBCORE-955: added LHCb git custom commands
New git commands:
- git lb-use <Project>
- add the default remote repository for the requested project
- git lb-checkout <Project>/<branch_or_tag> <directory>
- checkout a directory from the given project/branch
- git lb-push <Project> <new_branch>
- push the changes in the local repository for the directories taken from the project, to the new branch in the remote repository of the project