tools/impact: Use printf instead of echo -n

It turns out that on OS X, /bin/sh pretends it doesn't know the echo -n
option and just prints it with other input.

tools/impact

@@ -100,13 +100,13 @@ fi
 
 cd_to_root
 
-echo -n "Measuring commit $commit_before..."
+printf "Measuring commit %s..." "$commit_before"
 prepare "$commit_before"
 speed1=$(measure_speed)
 size1=$(measure_size)
 echo " OK"
 
-echo -n "Measuring commit $commit_after..."
+printf "Measuring commit %s..." "$commit_after"
 prepare "$commit_after"
 speed2=$(measure_speed)
 size2=$(measure_size)