support gerrit style refs/changes/... for branch names

To use a gerrit "branch" with devstack I would find the repo/remote and
set it for the specific project.

Example: https://review.openstack.org/2059

Would mean I update my localrc with the following settings:

KEYSTONE_REPO=https://review.openstack.org/p/openstack/keystone
KEYSTONE_BRANCH=refs/changes/59/2059/2

Change-Id: I0793415fb03cc08d1eb1a3faf1b8ec3e723beb31

stack.sh

@@ -448,10 +448,18 @@ function git_clone {
     GIT_DEST=$2
     GIT_BRANCH=$3
 
+    if echo $GIT_BRANCH | egrep -q "^refs"; then
+        # If our branch name is a gerrit style refs/changes/...
+        if [ ! -d $GIT_DEST ]; then
+            git clone $GIT_REMOTE $GIT_DEST
+        fi
+        cd $GIT_DEST
+        git fetch $GIT_REMOTE $GIT_BRANCH && git checkout FETCH_HEAD
+    else
         # do a full clone only if the directory doesn't exist
         if [ ! -d $GIT_DEST ]; then
             git clone $GIT_REMOTE $GIT_DEST
-        cd $2
+            cd $GIT_DEST
             # This checkout syntax works for both branches and tags
             git checkout $GIT_BRANCH
         elif [[ "$RECLONE" == "yes" ]]; then
@@ -469,6 +477,7 @@ function git_clone {
             git branch -D $GIT_BRANCH || true
             git checkout -b $GIT_BRANCH
         fi
+    fi
 }
 
 # compute service