Make test-verify SUSv3 compatible.

'head -c' isn't supported on (at least) OpenBSD and is not
part of SUSv3. Instead, use dd.

See also:

    http://www.opengroup.org/onlinepubs/009695399/utilities/head.html

tests/test-verify

@@ -35,7 +35,7 b' hg add a'
 hg ci -m a
 
 echo 'corrupted' > b
-head -c 20 .hg/store/data/a.i > start
+dd if=.hg/store/data/a.i of=start bs=1 count=20 2>/dev/null
 cat start b > .hg/store/data/a.i
 
 echo