Add pre-commit hook in content repo to catch dumb mistakes

[wincent]

Just accidentally created a file at /content/wiki/Ferret.vim instead of /content/wiki/Ferret.md. We should catch that kind of mistake with a hook.

Update: mitigated through use of script/new-article script.

[wincent]

Cross-referencing #50, which involves another potential dumb mistake (amending a published commit).

[wincent]

• pre-commit hook can check to see whether masochist:2:last-indexed-hash is an ancestor of HEAD (with git merge-base HEAD). This will prevent us from amending commits that have been indexed locally.
• amending already published history will result in a failure at git push time (unless you force), so that should stop mistakes propagating to the remote, even without a hook.

However, we can do better than that:

• pre-push hook can tag all pushes to content remote, so that we know when remote was last-indexed too.
• Can run our check logic against that too.