minor tweak to git hooks

- fix PREVIOUS_HEAD when run as git-merge
- catch fab css failure

git-hooks/post-checkout

@@ -1,17 +1,22 b''
 #!/bin/sh
 
 git submodule init
 git submodule update
 
-PREVIOUS_HEAD=$1
+if [[ "$(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
+  fab css || echo "failed to compile css"
 fi