Use 'printf %s $x' notation in t5401

Use 'printf %s $x' notation in t5401

We only care about getting what should be an empty string and
sending it to a file, without a trailing LF, so the empty string
translates into a 0 byte file.  Earlier when I originally wrote
these lines Mac OS X allowed the format string of printf to be
the empty string, but more recent versions appear to have been
'improved' with error messages if the format is not given.

This may cause problems if we ever wind up with changes to the hook
tests.  A minor cleanup makes the test more safe on all systems,
by conforming to accepted printf conventions.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>