test-lib: valgrind for only tests matching a pattern

With the new --valgrind-only=<pattern> option, one can enable
--valgrind at a per-test granularity, exactly analogous to
--verbose-only from the previous commit.

The options are wired such that --valgrind implies --verbose (as
before), but --valgrind-only=<pattern> implies
--verbose-only=<pattern> unless --verbose is also in effect.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/valgrind/valgrind.sh

@@ -4,6 +4,9 @@ base=$(basename "$0")
 
 TOOL_OPTIONS='--leak-check=no'
 
+test -z "$GIT_VALGRIND_ENABLED" &&
+exec "$GIT_VALGRIND"/../../"$base" "$@"
+
 case "$GIT_VALGRIND_MODE" in
 memcheck-fast)
 	;;