ktest.pl: Fix combined usage of BISECT_REVERSE and BISECT_SKIP

ktest.pl: Fix combined usage of BISECT_REVERSE and BISECT_SKIP

When BISECT_REVERSE and BISECT_SKIP are used together with boot or test
testing, build failures are treated as boot or test failures and
'git bisect bad' is executed instead of 'git bisect skip'. This is because
the $ret value of -1 is treated as a build failure, but the $reverse_bisect
logic does not properly handle this.

Simple fix, only invert it if it is positive.

Link: http://lkml.kernel.org/r/1335235380-8509-1-git-send-email-Russ.Dill@ti.com

Signed-off-by: Russ Dill <Russ.Dill@ti.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>