test-lib: provide test prereq REFFILES

REFFILES can be used to mark tests that are specific to the packed/loose ref
storage format and its limitations. Marking such tests is a preparation for
introducing the reftable storage backend.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -1114,6 +1114,12 @@ use these, and "test_set_prereq" for how to define your own.
 
    Git wasn't compiled with NO_PTHREADS=YesPlease.
 
+ - REFFILES
+
+   Test is specific to packed/loose ref storage, and should be
+   disabled for other ref storage backends
+
+
 Tips for Writing Tests
 ----------------------
 

t/test-lib.sh

@@ -1481,6 +1481,8 @@ parisc* | hppa*)
 	;;
 esac
 
+test_set_prereq REFFILES
+
 ( COLUMNS=1 && test $COLUMNS = 1 ) && test_set_prereq COLUMNS_CAN_BE_1
 test -z "$NO_PERL" && test_set_prereq PERL
 test -z "$NO_PTHREADS" && test_set_prereq PTHREADS