t/lib-httpd: pass GIT_TEST_SIDEBAND_ALL through Apache

07c3c2aa16 ("tests: define GIT_TEST_SIDEBAND_ALL", 2019-01-16) added
GIT_TEST_SIDEBAND_ALL to the apache.conf PassEnv list.  Avoid warnings
from Apache when the variable is unset, as we do for GIT_VALGRIND* and
GIT_TRACE, from f628825481 ("t/lib-httpd: handle running under
--valgrind", 2012-07-24) and 89c57ab3f0 ("t: pass GIT_TRACE through
Apache", 2015-03-13), respectively.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/lib-httpd.sh b/t/lib-httpd.sh
index a8729f8..3eb665f 100644
--- a/t/lib-httpd.sh
+++ b/t/lib-httpd.sh
@@ -91,6 +91,7 @@
 # hack to suppress apache PassEnv warnings
 GIT_VALGRIND=$GIT_VALGRIND; export GIT_VALGRIND
 GIT_VALGRIND_OPTIONS=$GIT_VALGRIND_OPTIONS; export GIT_VALGRIND_OPTIONS
+GIT_TEST_SIDEBAND_ALL=$GIT_TEST_SIDEBAND_ALL; export GIT_TEST_SIDEBAND_ALL
 GIT_TRACE=$GIT_TRACE; export GIT_TRACE
 
 if ! test -x "$LIB_HTTPD_PATH"