thread-utils.c: detect online CPU count on OpenBSD / NetBSD

OpenBSD / NetBSD use HW_NCPUONLINE to detect the online CPU
count. OpenBSD ships with SMT disabled on X86 systems so
HW_NCPU would provide double the number of CPUs as opposed
to the proper online count.

Signed-off-by: Brad Smith <brad@comstyle.com>
Reviewed-by: Collin Funk <collin.funk1@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

thread-utils.c

diff --git a/thread-utils.c b/thread-utils.c
index 1f89ffa..374890e 100644
--- a/thread-utils.c
+++ b/thread-utils.c
@@ -46,11 +46,11 @@ int online_cpus(void)
 	mib[0] = CTL_HW;
 #  ifdef HW_AVAILCPU
 	mib[1] = HW_AVAILCPU;
-	len = sizeof(cpucount);
-	if (!sysctl(mib, 2, &cpucount, &len, NULL, 0))
-		return cpucount;
-#  endif /* HW_AVAILCPU */
+#  elif defined(HW_NCPUONLINE)
+	mib[1] = HW_NCPUONLINE;
+#  else
 	mib[1] = HW_NCPU;
+#  endif /* HW_AVAILCPU */
 	len = sizeof(cpucount);
 	if (!sysctl(mib, 2, &cpucount, &len, NULL, 0))
 		return cpucount;