Try harder to stop the branch builds.

.appveyor.yml

@@ -23,9 +23,25 @@ install:
 # We have a custom "build" system.  We don't need MSBuild or whatever.
 build: off
 
-# Do not build feature branch with open Pull Requests
+# Do not build feature branch with open pull requests.  This is documented but
+# it's not clear it does anything.
 skip_branch_with_pr: true
 
+# This, perhaps, is effective.
+branches:
+  # whitelist
+  only:
+    - 'master'
+
+skip_commits:
+ files:
+   # The Windows builds are unaffected by news fragments.
+   - 'newsfragments/*'
+   # Also, all this build junk.
+   - '.circleci/*'
+   - '.lgtm.yml'
+   - '.travis.yml'
+
 # we run from C:\projects\tahoe-lafs
 
 test_script: