t7900: create commit so that branch is born

`pack-refs task` cannot be run in isolation but does pass if
`maintenance.auto config option` is run first.

Create a commit so that `HEAD` does not point to an unborn branch.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7900-maintenance.sh

@@ -388,6 +388,7 @@ test_expect_success 'maintenance.incremental-repack.auto (when config is unset)'
 '
 
 test_expect_success 'pack-refs task' '
+	test_commit message &&
 	for n in $(test_seq 1 5)
 	do
 		git branch -f to-pack/$n HEAD || return 1