Merge branch 'js/maint-1.6.6-send-pack-stateless-rpc-deadlock-fix' into js/maint-send-pack-stateless-rpc-deadlock-fix

* js/maint-1.6.6-send-pack-stateless-rpc-deadlock-fix: send-pack: avoid deadlock when pack-object dies early Evil merge to adjust the way the use of pthreads in sideband-demultiplexor was decided (earlier it was "if we are not on Windows", now it is "if we are not using pthreads").