Fixed git show for upper-constraints.txt in lib/tempest

Change-Id: Id133108e0436018be81fca74a15ff29a4fe2e796
Closes-Bug: #1789643

Author: Lenny Verkhovsky

lib/tempest

@@ -560,7 +560,7 @@ function configure_tempest {
     fi
 
     # The requirements might be on a different branch, while tempest needs master requirements.
-    (cd $REQUIREMENTS_DIR && git show master:upper-constraints.txt) > u-c-m.txt
+    (cd $REQUIREMENTS_DIR && git show origin/master:upper-constraints.txt) > u-c-m.txt
     tox -evenv-tempest -- pip install -c u-c-m.txt -r requirements.txt
 
     # Auth: