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

@@ -1,45 +1,45 b''
 #!/bin/sh
 
 echo % prepare repo
 hg init a
 cd a
 echo "some text" > FOO.txt
 echo "another text" > bar.txt
 echo "more text" > QUICK.txt
 hg add
 hg ci -mtest1
 
 echo
 echo % verify
 hg verify
 
 echo
 echo % introduce some bugs in repo
 cd .hg/store/data
 mv _f_o_o.txt.i X_f_o_o.txt.i
 mv bar.txt.i xbar.txt.i
 rm _q_u_i_c_k.txt.i
 
 echo
 echo % verify
 hg verify
 
 cd ..
 
 echo % test revlog corruption
 hg init b
 cd b
 
 touch a
 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
 echo % verify
 hg verify
 
 exit 0