t: simplify loop exit-code status variables

t: simplify loop exit-code status variables

Since shell loops may drop the exit code of failed commands
inside the loop, some tests try to keep track of the status
by setting a variable. This can end up cumbersome and hard
to read; it is much simpler to just exit directly from the
loop using "return 1" (since each case is either in a helper
function or inside a test snippet).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>