use posix compliant option for head

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

tests/test-grep

@@ -14,7 +14,7 b" 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