Use upper-constraints from in review changes

Currently upper-constraints.txt is not getting used
from in-review changes of requirements project and
leading to merge of broken requirements[1].

Use master branch to fetch constraints instead of
the remote branch.

[1] https://review.opendev.org/c/openstack/requirements/+/822575

Depends-On: https://review.opendev.org/c/openstack/requirements/+/823128
Change-Id: I5d42ac6b54bf20804d7e5faa39d1289102318b64
diff --git a/lib/tempest b/lib/tempest
index ab80217..bdbd3ca 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -119,7 +119,7 @@
     local tmp_c
     tmp_c=$1
     if [[ $TEMPEST_VENV_UPPER_CONSTRAINTS == "master" ]]; then
-        (cd $REQUIREMENTS_DIR && git show origin/master:upper-constraints.txt) > $tmp_c
+        (cd $REQUIREMENTS_DIR && git show master:upper-constraints.txt) > $tmp_c
     else
         echo "Using $TEMPEST_VENV_UPPER_CONSTRAINTS constraints in Tempest virtual env."
         cat $TEMPEST_VENV_UPPER_CONSTRAINTS > $tmp_c