Fix warnings spam

Task: PRODX-54485

Change-Id: Ic4868cc29435e4cd1533e737ce8eb8ca8fa27c13
diff --git a/src/com/mirantis/mk/Git.groovy b/src/com/mirantis/mk/Git.groovy
index eb08e7d..74571eb 100644
--- a/src/com/mirantis/mk/Git.groovy
+++ b/src/com/mirantis/mk/Git.groovy
@@ -64,6 +64,7 @@
  * @param branch          Branch desired to switch to
  */
 def changeGitBranch(path, branch) {
+    def git_cmd
     dir(path) {
         git_cmd = sh (
             script: "git checkout ${branch}",