Move sorting to SortedSelectMixin to allow child classes to override it

Signed-off-by: Michal Čihař <michal@cihar.com>
This should raise only IndexError

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

Signed-off-by: Michal Čihař <michal@cihar.com>
Make is_git_lockable an attribute

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