test-lib: don't use ulimit in test prerequisites on cygwin

On cygwin (and MinGW), the 'ulimit' built-in bash command does not have
the desired effect of limiting the resources of new processes, at least
for the stack and file descriptors. However, it always returns success
and leads to several test prerequisites being erroneously set to true.

Add a check for cygwin and MinGW to the prerequisite expressions, using
a 'test_have_prereq !MINGW,!CYGWIN' clause, to guard against using ulimit.
This affects the prerequisite expressions for the ULIMIT_STACK_SIZE,
CMDLINE_LIMIT and ULIMIT_FILE_DESCRIPTORS prerequisites.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1400-update-ref.sh

@@ -1253,7 +1253,10 @@ run_with_limited_open_files () {
 	(ulimit -n 32 && "$@")
 }
 
-test_lazy_prereq ULIMIT_FILE_DESCRIPTORS 'run_with_limited_open_files true'
+test_lazy_prereq ULIMIT_FILE_DESCRIPTORS '
+	test_have_prereq !MINGW,!CYGWIN &&
+	run_with_limited_open_files true
+'
 
 test_expect_success ULIMIT_FILE_DESCRIPTORS 'large transaction creating branches does not burst open file limit' '
 (

t/t6120-describe.sh

@@ -279,7 +279,6 @@ test_expect_success 'describe ignoring a borken submodule' '
 	grep broken out
 '
 
-# we require ulimit, this excludes Windows
 test_expect_failure ULIMIT_STACK_SIZE 'name-rev works in a deep repo' '
 	i=1 &&
 	while test $i -lt 8000

t/t7004-tag.sh

@@ -1863,7 +1863,6 @@ test_expect_success 'version sort with very long prerelease suffix' '
 	git tag -l --sort=version:refname
 '
 
-# we require ulimit, this excludes Windows
 test_expect_success ULIMIT_STACK_SIZE '--contains and --no-contains work in a deep repo' '
 	>expect &&
 	i=1 &&

t/test-lib.sh

@@ -1165,13 +1165,19 @@ run_with_limited_cmdline () {
 	(ulimit -s 128 && "$@")
 }
 
-test_lazy_prereq CMDLINE_LIMIT 'run_with_limited_cmdline true'
+test_lazy_prereq CMDLINE_LIMIT '
+	test_have_prereq !MINGW,!CYGWIN &&
+	run_with_limited_cmdline true
+'
 
 run_with_limited_stack () {
 	(ulimit -s 128 && "$@")
 }
 
-test_lazy_prereq ULIMIT_STACK_SIZE 'run_with_limited_stack true'
+test_lazy_prereq ULIMIT_STACK_SIZE '
+	test_have_prereq !MINGW,!CYGWIN &&
+	run_with_limited_stack true
+'
 
 build_option () {
 	git version --build-options |