SourceContext: add same_project comparison

The SourceContext equality check is using the in repository yaml path
which may differ for a single project, resulting in secrets being
incorrectly validated when they are defined in dedicated files.

This change adds a same_project method to validate secrets.

Change-Id: I5500e43faa3cbb7ed470575fe54cb66aed343b9a
diff --git a/zuul/model.py b/zuul/model.py
index 0e42368..4c5a51f 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -627,6 +627,13 @@
         return self.__class__(self.project, self.branch, self.path,
                               self.trusted)
 
+    def isSameProject(self, other):
+        if not isinstance(other, SourceContext):
+            return False
+        return (self.project == other.project and
+                self.branch == other.branch and
+                self.trusted == other.trusted)
+
     def __ne__(self, other):
         return not self.__eq__(other)