`--help` not working when used as a git utility

[tommarshall]

The --help argument is not working correctly when the script is used as a git utility:

$ git diff-ansible-vault --help
No manual entry for git-diff-ansible-vault

It works fine with -h, or when the full script name is used:

$ git diff-ansible-vault -h
Usage: git-diff-ansible-vault [-r <revision>] [-p <path>] [options]
...

$ git-diff-ansible-vault --help
Usage: git-diff-ansible-vault [-r <revision>] [-p <path>] [options]
...