Remove the explicit `--revision` & `--path` arguments

[tommarshall]

Currently git-diff-ansible-vault requires the revision and path to be explicitly specified with arguments, e.g.

$ git diff-ansible-vault -r master..some-branch -p some/path/to/file

It would be better to have parity with git-diff's revision and path arguments, so that you could jump straight from diff to diff-ansible-vault without having to add extra flags.

From git-diff's man page:

       git diff [--options] [--] [<path>...]

       git diff [--options] <commit> [--] [<path>...]

       git diff [--options] <commit> <commit> [--] [<path>...]

       git diff [--options] <commit>..<commit> [--] [<path>...]

       git diff [--options] <commit>...<commit> [--] [<path>...]

       git diff [options] <blob> <blob>