t9300: avoid short reads from dd

t9300: avoid short reads from dd

dd is a thin wrapper around read(2).  As open group Issue 7 explains:

	It shall read the input one block at a time, using the specified
	input block size; it shall then process the block of data
	actually returned, which could be smaller than the requested
	block size.

Any short read --- for example from a pipe whose capacity cannot fill
a block --- results in that block being truncated.  As a result, the
first cat-blob test (9300.114) fails on Mac OS X, where the pipe
capacity is around 8 KiB.

Fix the test by using a block size of 1.  Each read will block until
the next byte of input is available.

It would be even nicer to use head -c which expresses the intention
more clearly.  Alas, IRIX "head" does not support the -c option.

Reported-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>