Fix install-hooks.bash script

I hadn’t realized there were Git hooks, but noticed them when I was
adding the Weeder script.

So, I documented that they exist, and then fixed the script since it
wasn’t working for me.

It now works
- when run it from anywhere in the repo (not just from two directories
  down somewhere),
- when `core.hooksPath` is set (i.e., `$GIT_DIR/hooks` isn’t the right
  place),
- when run from a worktree (which doesn’t necessarily have its own hooks
  dir), and
- when the hooks directory doesn’t exist.

Author: sellout

development.markdown

@@ -20,6 +20,14 @@ To get cracking with Unison:
 
 On startup, Unison prints a url for the codebase UI. If you did step 3 above, then visiting that URL in a browser will give you a nice interface to your codebase.
 
+## Git hooks
+
+There are some Git hooks provided by Unison. If you want to use them, you can run
+
+``` bash
+./scripts/install-hooks.bash
+```
+
 ## Autoformatting your code with Ormolu
 
 We use Ormolu (see [the specific version](./nix/versions.nix)) and CI will add an extra commit, if needed, to autoformat your code.

scripts/install-hooks.bash

@@ -1,9 +1,12 @@
 #!/usr/bin/env bash
+set -euo pipefail
+SCRIPTDIR="$(dirname -- "$(readlink -f -- "${BASH_SOURCE[0]}")")"
 
-GIT_DIR=$(git rev-parse --git-dir)
+hooks_dir=$(git rev-parse --git-path hooks)
 
 echo "Installing hooks..."
+mkdir "$hooks_dir"
 # this command creates symlink to our pre-commit script
-ln -s ../../scripts/pre-commit.bash $GIT_DIR/hooks/pre-commit
-ln -s ../../scripts/pre-push.bash $GIT_DIR/hooks/pre-push
+ln -s "$SCRIPTDIR/pre-commit.bash" "$hooks_dir/pre-commit"
+ln -s "$SCRIPTDIR/pre-push.bash" "$hooks_dir/pre-push"
 echo "Done!"