add support for calling the hook from a symlink

I setup your pre-commit hook as a symlink in my puppet git repo, and got this error:

```
#!/bin/bash
.git/hooks/pre-commit: line 39: /home/gpetras/git/hieradata/.git/hooks/commit_hooks/yaml_syntax_check.sh: No such file or directory
.git/hooks/pre-commit: line 65: /home/gpetras/git/hieradata/.git/hooks/commit_hooks/rspec_puppet_checks.sh: No such file or directory
Error: 2 subhooks failed. Aborting commit.
```

Thought you might be interested in the fix that worked for me.

Thanks for the hooks!

Greg