Use override-checkout to select jobs

If a job is defined in a project with "master" and "stable" branches,
and a user wants to inherit from that job in a project with only a
"devel" branch, Zuul will not run the job because there are no matching
variants.  To correct this, have Zuul use the value of override-checkout
to mask the branch of the item under test so that it appears to match the
'override-checkout' branch.

Change-Id: I4619a6f9fe9c3203a7285e5eb4f74c52b650c737
diff --git a/tests/fixtures/config/branch-mismatch/main.yaml b/tests/fixtures/config/branch-mismatch/main.yaml
new file mode 100644
index 0000000..950b117
--- /dev/null
+++ b/tests/fixtures/config/branch-mismatch/main.yaml
@@ -0,0 +1,9 @@
+- tenant:
+    name: tenant-one
+    source:
+      gerrit:
+        config-projects:
+          - common-config
+        untrusted-projects:
+          - org/project1
+          - org/project2