use posix compliant option for head

head -N is not posix compliant, head -n N should be used instead

tests/test-grep

@@ -1,23 +1,23 b''
 #!/bin/sh
 
 mkdir t
 cd t
 hg init
 echo import > port
 hg add port
 hg commit -m 0 -u spam -d '0 0'
 echo export >> port
 hg commit -m 1 -u eggs -d '1 0'
 echo export > port
 echo vaportight >> port
 echo 'import/export' >> port
 hg commit -m 2 -u spam -d '2 0'
 echo 'import/export' >> port
 hg commit -m 3 -u eggs -d '3 0'
-head -3 port > port1
+head -n 3 port > port1
 mv port1 port
 hg commit -m 4 -u spam -d '4 0'
 hg grep port port
 echo 'FIXME: history is wrong here'
 hg grep --all -nu port port
 hg grep import port