lstat_cache(): print a warning if doing ping-pong between cache types

lstat_cache(): print a warning if doing ping-pong between cache types

This is a debug patch which is only to be used while the lstat_cache()
is in the test stage, and should be removed/reverted before the final
relase.

I think it should be useful to catch these warnings, as I it could be
an indication of that the cache would not be very effective if it is
doing ping-pong by switching between different cache types too many
times.

Also, if someone is experimenting with the lstat_cache(), this patch
will maybe be useful while debugging.

If someone is able to trigger the warning, then send a mail to the GIT
mailing list, containing the first 15 lines of the warning, and a
short description of the GIT commands to trigger the warnings.

I hope someone is willing to use this patch for a while, to be able to
catch possible ping-pong's.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>