[CALL FOR TESTING] Cygwin-3.6.0

[Jeremy Drake]

I rebased msys2's msys2-runtime fork of cygwin on top of cygwin's master
branch, and then rebased git-for-windows' msys2-runtime on top of THAT.

1. Git for Windows's test suite still passes with that (which is nice, it
tended to hang when we did this with 3.5.5).

2. This seems to have fixed a severe performance regression in Curl's test
suite (https://github.com/curl/curl/pull/16217) that I was recently
looking into.  My research tracked it down to c7fe29f ("Cygwin: pipe:
Restore blocking mode of read pipe on close()/raw_read()"), reverting that
got performance back to normal.  The fhandler/pipe.cc code changed
significantly in master vs 3.5.