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>

t/t9300-fast-import.sh

@@ -1794,7 +1794,7 @@ test_expect_success PIPE 'R: copy using cat-file' '
 
 	read blob_id type size <&3 &&
 	echo "$blob_id $type $size" >response &&
-	dd of=blob bs=$size count=1 <&3 &&
+	dd of=blob bs=1 count=$size <&3 &&
 	read newline <&3 &&
 
 	cat <<EOF &&
@@ -1845,7 +1845,7 @@ test_expect_success PIPE 'R: print blob mid-commit' '
 		EOF
 
 		read blob_id type size <&3 &&
-		dd of=actual bs=$size count=1 <&3 &&
+		dd of=actual bs=1 count=$size <&3 &&
 		read newline <&3 &&
 
 		echo
@@ -1880,7 +1880,7 @@ test_expect_success PIPE 'R: print staged blob within commit' '
 		echo "cat-blob $to_get" &&
 
 		read blob_id type size <&3 &&
-		dd of=actual bs=$size count=1 <&3 &&
+		dd of=actual bs=1 count=$size <&3 &&
 		read newline <&3 &&
 
 		echo deleteall