Fix checkout of refspec

Change-Id: I75491b5e74cb06683b993291478a7245fce9fd52
Related-Prod: PRODX-3456
diff --git a/src/com/mirantis/mcp/Git.groovy b/src/com/mirantis/mcp/Git.groovy
index 4231016..53efa09 100644
--- a/src/com/mirantis/mcp/Git.groovy
+++ b/src/com/mirantis/mcp/Git.groovy
@@ -79,7 +79,7 @@
   def credentialsId = config.get('credentialsId', '')
   def protocol = config.get('protocol', 'ssh')
   def refspec = config.get('refspec', null)
-  String branch = config.get('branch', 'FETCH_HEAD')
+  String branch = refspec ? 'FETCH_HEAD' : "*/${config.get('branch')}"
   Integer depth = config.get('depth', 0)
   Integer timeout = config.get('timeout', 0)