[PATCH] pre-commit sample hook: do not barf on the initial import

The example hook barfs on the initial import.  Ideally it should
produce a diff from an empty tree, but for now let's stop at
squelching the bogus error message.  Often an initial import
involves tons of badly formatted files from foreign SCM, so not
complaining about them like this patch does might actually be a
better idea than enforcing the "Perfect Patch" format on them.

Signed-off-by: Junio C Hamano <junkio@cox.net>