test-lib: --valgrind should not override --verbose-log

The --verbose test option cannot be used with test harnesses
like "prove". Instead, you must use --verbose-log.

Since the --valgrind option implies --verbose, that means
that it cannot be used with prove. I.e., this does not work:

  prove t0000-basic.sh :: --valgrind

You'd think it could be fixed by doing:

  prove t0000-basic.sh :: --valgrind --verbose-log

but that doesn't work either, because the implied --verbose
takes precedence over --verbose-log. If the user has given
us a specific option, we should prefer that.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -274,7 +274,7 @@ then
 	test -z "$verbose" && verbose_only="$valgrind_only"
 elif test -n "$valgrind"
 then
-	verbose=t
+	test -z "$verbose_log" && verbose=t
 fi
 
 if test -n "$color"