add CLI support for computing the git hash of a folder

It seems that there is almost support here for computing the git hash of a folder.

For instance, the CLI would be

./git-commit-hash .
040000 tree 30acfefacc9c0daa7efaf8ac7c622517ef0420f1	.
./git-commit-hash a_folder
040000 tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904	

This is a very useful feature that is hard to find. The only other implementation I know is https://github.com/chris3torek/scripts/blob/master/githash.py but it is slow.

Thanks!