Merge branch 'rj/test-ulimit-on-windows' into next

On Cygwin, "ulimit -s" does not report failure but it does not work
at all, which causes an unexpected success of some tests that
expect failures under a limited stack situation.  This has been
fixed.

* rj/test-ulimit-on-windows:
t9010-*.sh: skip all tests if the PIPE prereq is missing
test-lib: use more compact expression in PIPE prerequisite
test-lib: don't use ulimit in test prerequisites on cygwin

diff --cc t/t6120-describe.sh
Simple merge

diff --cc t/test-lib.sh
Simple merge