Fix pre-commit, prepare-commit-msg hooks

For users who had not overridden the `pre-commit` or
`prepare-commit-msg` Git hooks,
they would see `git commit` hang,
with nothing happening.

The command would exit without going to their editor.

The bug was introduced in d7f194f94ee1ab535da18e657f0c51479c0233b4
and the fix is to return a 0 exit code when the override is not present.