Use libgit2 with threads

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Do not warn on an unclean work tree when squashing commits

Fixes a regression of a0507c306889bdb1ee92c0f84986085c8132621e.

Signed-off-by: Sven Strickroth <email@cs-ware.de>