Add implied-branches pragma directive

When two projects have dissimilar stable branch names, for example,
stable/pike and stable/jewel, but should generally be used together
and therefore share job variants, it can be difficult to make that
happen.  Currently, one must add explicit multi-branch matchers
to every such job and project-template.

This allows a user to add a pragma directive to indicate all the
jobs in a file should apply to multiple branches.

Change-Id: I57cf159992d8f501cbaf41aef19562951ef6b7ea
diff --git a/tests/fixtures/config/pragma-multibranch/main.yaml b/tests/fixtures/config/pragma-multibranch/main.yaml
new file mode 100644
index 0000000..950b117
--- /dev/null
+++ b/tests/fixtures/config/pragma-multibranch/main.yaml
@@ -0,0 +1,9 @@
+- tenant:
+    name: tenant-one
+    source:
+      gerrit:
+        config-projects:
+          - common-config
+        untrusted-projects:
+          - org/project1
+          - org/project2