Fix tempest upper-constraints

When deploying devstack in a stable branch, the master branch is
available locally only in a CI environment where Zuul prepares all
available branches. For a non-CI deployment we need to stick to using
the remote branch as was the case before [0].

While the situation on the master branch isn't really broken, we apply
the fix here anyway so that future stable branches are created in a
working state.

[0] I5d42ac6b54bf20804d7e5faa39d1289102318b64

Closes-Bug: #1956219
Signed-off-by: Dr. Jens Harbott <harbott@osism.tech>
Change-Id: Ib7719cb2d48b34db70f885e0afe77d904abba3b5
diff --git a/lib/tempest b/lib/tempest
index bdbd3ca..adffeda 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -119,7 +119,9 @@
     local tmp_c
     tmp_c=$1
     if [[ $TEMPEST_VENV_UPPER_CONSTRAINTS == "master" ]]; then
-        (cd $REQUIREMENTS_DIR && git show master:upper-constraints.txt) > $tmp_c
+        (cd $REQUIREMENTS_DIR &&
+            git show master:upper-constraints.txt 2>/dev/null ||
+            git show origin/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