t1800: correct test to handle Cygwin

On Cygwin, when failing to spawn a process using start_command, Git
outputs the same error as on Linux systems, rather than using the
GIT_WINDOWS_NATIVE-specific error output.  The WINDOWS test prerequisite
is set in both Cygwin and native Windows environments, which means it's
not appropriate to use to anticipate the error output from
start_command.  Instead, use the MINGW test prerequisite, which is only
set for Git in native Windows environments, and not for Cygwin.

Signed-off-by: Adam Dinwoodie <adam@dinwoodie.org>
Helped-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1800-hook.sh

@@ -157,9 +157,9 @@ test_expect_success 'git hook run a hook with a bad shebang' '
 	write_script bad-hooks/test-hook "/bad/path/no/spaces" </dev/null &&
 
 	# TODO: We should emit the same (or at least a more similar)
-	# error on Windows and !Windows. See the OS-specific code in
-	# start_command()
-	if test_have_prereq !WINDOWS
+	# error on MINGW (essentially Git for Windows) and all other
+	# platforms.. See the OS-specific code in start_command()
+	if test_have_prereq !MINGW
 	then
 		cat >expect <<-\EOF
 		fatal: cannot run bad-hooks/test-hook: ...