limit the number of affected packages mentioned in MR discussion

For repository-wide changes, many packages are affected. If the list
of affected packages becomes very long, the automatic comment from
the atlasbot does not really help, but only makes following the
discussion on the Gitlab web interface more difficult. Therefore,
the list of affected packages is only printed for at most 20 packages.

Closes ATLINFR-1488

CI/handle_new_mr.py

@@ -11,11 +11,16 @@ def comment_affected_packages(packages):
 
     return: comment text
     """
-    if len(packages) > 0:
-        comment = "This merge request affects %d package%s:  \n- " % (len(packages),'' if len(packages) == 1 else 's')
+    n_packages = len(packages)
+    if n_packages == 0:
+        comment = "This merge request affects no known packages. Consult an expert!"
+    elif n_packages == 1:
+        comment = "This merge request affects 1 package:  \n- " + packages.pop()
+    elif n_packages <= 20:
+        comment = "This merge request affects %d packages:  \n- " % n_packages
         comment += "  \n- ".join(sorted(packages))
     else:
-        comment = "This merge request affects no known packages. Consult an expert!"
+        comment = "This merge request affects %d packages. Since this is a long list, I will not print it here." % n_packages
 
     return comment