run commit hook on what we're committing (#356)

(rather than what's in our WC)

hooks/pre-commit

@@ -2,4 +2,21 @@
 
 set -eu
 
+# make the GIT_DIR and GIT_INDEX_FILE absolute, before we change dir
+export GIT_DIR=$(readlink -f `git rev-parse --git-dir`)
+if [ -n "${GIT_INDEX_FILE:+x}" ]; then
+    export GIT_INDEX_FILE=$(readlink -f "$GIT_INDEX_FILE")
+fi
+
+# create a temp dir. The `trap` incantation will ensure that it is removed
+# again when this script completes.
+tmpdir=`mktemp -d`
+trap 'rm -rf "$tmpdir"' EXIT
+cd "$tmpdir"
+
+# get a clean copy of the index (ie, what has been `git add`ed), so that we can
+# run the checks against what we are about to commit, rather than what is in
+# the working copy.
+git checkout-index -a
+
 ./scripts/find-lint.sh fast