git-whatchanged: use the git-rev-parse helper

So now you can say

	git-whatchanged -p v2.6.12-rc5.. drivers/usb

and it will show you what changed (as a patch) in drivers/usb since the
v2.6.12-rc5 release.

git-whatchanged

 #!/bin/sh
-git-rev-list HEAD | git-diff-tree --stdin --pretty -r "$@" | LESS="$LESS -S" ${PAGER:-less}
+git-rev-list $(git-rev-parse --default HEAD --revs-only "$@") |
+	git-diff-tree --stdin --pretty -r $(git-rev-parse --no-revs "$@") |
+	LESS="$LESS -S" ${PAGER:-less}