gerrit tests should return to checked out branch

when running "tox -e gerrit" on a local checkout
after the tests have finshed the repo is checked out
to the HEAD commit, not the branch that was checked
out before the run.

This ensures the branch is checked out at the end
of the test.

Change-Id: I58dae937000d05c3a9b09dc4aed5362a5c56d133

tools/check_gerrit_projects_changed.sh

@@ -2,6 +2,7 @@
 
 
 GITHEAD=$(git rev-parse HEAD)
+GITBRANCH=$(git rev-parse --abbrev-ref HEAD)
 
 # Check out previous version
 git checkout HEAD~1
@@ -16,3 +17,4 @@ python tools/check_gerrit_projects_changed.py gerrit/projects-old.yaml \
     gerrit/projects.yaml
 
 rm gerrit/projects-old.yaml
+git checkout $GITBRANCH