dim: assert_branch before pull requests

In the per-branch checkout dir model assert_branch will simply do a
cd.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

dim

@@ -429,6 +429,8 @@ function dim_pull_request
 	branch=$1
 	upstream=$2
 
+	assert_branch $branch
+
 	git fetch ${upstream%%/*} >& /dev/null || true
 	echo "Using $upstream as the upstream"