Git hooks use bash specific syntax

git-hooks/post-checkout

@@ -1,22 +1,22 b''
-#!/bin/sh
+#!/bin/bash
 
 git submodule init
 git submodule update
 
 if [[ "$(basename $0)" == "post-merge" ]]; then
   PREVIOUS_HEAD=ORIG_HEAD
 else
   PREVIOUS_HEAD=$1
 fi
 
 # if style changed (and less/fabric available), rebuild sourcemaps
 if [[
   ! -z "$(git diff $PREVIOUS_HEAD IPython/html/static/style/ipython.min.css)"
   && ! -z "$(git diff $PREVIOUS_HEAD IPython/html/static/style/style.min.css)"
   && ! -z $(which 2>/dev/null lessc)
   && ! -z $(which 2>/dev/null fab)
 ]]; then
   echo "rebuilding sourcemaps"
   cd IPython/html
   fab css || echo "failed to compile css"
 fi