fix: Raise an error if no refspec is provided

Only local repositories may be configured without refspec.
Raise an error if a repository url is given but refspec is not.

Signed-off-by: Tobias Schaffner <tobias.schaffner@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tests/test_refspec/test4.yml

@@ -0,0 +1,8 @@
+header:
+  version: 8
+
+repos:
+  this:
+
+  kas:
+    url: https://github.com/siemens/kas.git