Make "git log" use the new git-rev-parse helper

See the previous commit for explanations.

git-log-script

@@ -1,2 +1,2 @@
 #!/bin/sh
-git-rev-list --pretty HEAD | LESS=-S ${PAGER:-less}
+git-rev-list --pretty $(git-rev-parse --default HEAD "$@") | LESS=-S ${PAGER:-less}