Remove unused imports

Signed-off-by: Michal Čihař <michal@cihar.com>
Resolve symlinks when getting object hash

Signed-off-by: Michal Čihař <michal@cihar.com>
Use own git wrapper for manipulating git

Signed-off-by: Michal Čihař <michal@cihar.com>
Use own VCS abstraction for describing version

Signed-off-by: Michal Čihař <michal@cihar.com>
Remove GitPython from requirements

It is no longer needed as we have own abstraction layer.

Signed-off-by: Michal Čihař <michal@cihar.com>