util: Refactor resolving git url
    
    Split out a smaller function that receives a git repo URL and ref and
    returns the commit id. The caching resolver class is modified to use
    this second function if branch is given to it.
    
    The new function checks if the ref received already looks like a commit
    ID, and if so it does not attempt to do anything with it.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>