script/install-precommit: ask git where hooks live (closes #129)

The script wrote the hook to .git/hooks, which fails when .git is a
file rather than a directory, as in a clone made with
--separate-git-dir. It now asks git for the hooks directory with
`git rev-parse --git-path hooks` and creates it if missing. In an
ordinary clone git answers .git/hooks, so nothing moves there. Where
core.hooksPath is set, git answers that directory, so the hook now
lands where git runs it instead of in .git/hooks, where it never ran.

Model: opus-5-5

script/install-precommit

@@ -7,7 +7,11 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    hook=".git/hooks/pre-commit"
+    # Ask git where hooks live: .git is a file, not a directory, in some
+    # checkouts (for example a clone made with --separate-git-dir).
+    hooks="$(git rev-parse --git-path hooks)"
+    mkdir -p "$hooks"
+    hook="$hooks/pre-commit"
     printf '#!/bin/sh\nset -e\nscript/precommit\n' > "$hook"
     chmod +x "$hook"
     echo "pre-commit hook installed: runs script/precommit"