Support viewing commits from a specific commit hash
    
    With this commit we're allowing to view the list of commits from a
    specified commit instead of enforcing this be a branch name.
    This allows fixing the workflow: view release, view the corresponding
    tree, view the commits from there.
    
    Fixes https://pagure.io/pagure/issue/3190
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>