ktest: Add BISECT_FILES to run git bisect on paths

Add the config option BISECT_FILES that allows the user to
specify what path in the kernel to run the git bisect on.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

tools/testing/ktest/sample.conf

@@ -528,6 +528,15 @@
 #   when something other than the BISECT_TYPE fails, ktest.pl will
 #   run "git bisect skip" and try again.
 #
+# BISECT_FILES = <path> (optional, default undefined)
+#
+#   To just run the git bisect on a specific path, set BISECT_FILES.
+#   For example:
+#
+#     BISECT_FILES = arch/x86 kernel/time
+#
+#   Will run the bisect with "git bisect start -- arch/x86 kernel/time"
+#
 # BISECT_REVERSE = 1 (optional, default 0)
 #
 #   In those strange instances where it was broken forever