Merge branch 'jk/push-client-deadlock-fix'

Some Windows SDK lacks pthread_sigmask() implementation and fails
to compile the recently updated "git push" codepath that uses it.

* jk/push-client-deadlock-fix:
Windows: only add a no-op pthread_sigmask() when needed
Windows: add pthread_sigmask() that does nothing

diff --cc compat/mingw.h
Simple merge

diff --cc compat/win32/pthread.h
Simple merge