t/test-lib.sh: exit with small negagive int is ok with test_must_fail

The test_must_fail function in test-lib.sh has been designed to distinguish segmentation faults from controlled errors. But in the current implementation this only works if a git command does not return a small negative value, like -1, -2 or -3. But some git commands do. Because any signal (like SIGSEGV) will result in an exit status less than 193, this patch just adds a further check for the exit status. Signed-off-by: Stephan Beyer <s-beyer@gmx.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>