git-whatchanged: Add usage string

Signed-off-by: NFredrik Kuivinen <freku045@student.liu.se>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-whatchanged.sh

 #!/bin/sh
+
+USAGE='[-p] [--max-count=<n>] [<since>..<limit>] [--pretty=<format>] [-m] [git-diff-tree options] [git-rev-list options]'
+SUBDIRECTORY_OK='Yes'
+. git-sh-setup
+
 rev_list_args=$(git-rev-parse --sq --default HEAD --revs-only "$@") &&
 diff_tree_args=$(git-rev-parse --sq --no-revs "$@") &&