Don't fail with EEXISTS if symlinks already exist and point to the right places

Steps to repro:
1. `cd` to a git repo containing a node.js project
1. `npm i --save-dev shared-git-hooks` in a git repo that's never had `shared-git-hooks`
1. Observe correct symlinks created in `.git/hooks`
1. `rm -rf node_modules`
1. `npm i`
1. Observe `EEXISTS` errors

If the symlinks exist & point to the correct locations for `shared-git-hooks`, don't fail, either warn or continue silently. If as few as one of the symlinks points to the wrong place, fail, or just warn.

Reproduce the issue using the npm install sequence and inspect the symlinks in .git/hooks after removing node_modules. Done means reinstalling shared-git-hooks succeeds when existing symlinks point to the correct locations, while incorrect symlinks produce the requested failure or warning behavior.

Written by the indexing model from the issue text.