Fix get-changed-servers in CI. (github#137)

Author: cmrigney

.github/workflows/ci.yaml

@@ -25,7 +25,7 @@ jobs:
         shell: bash
         run: |
           git fetch origin ${{ github.event.pull_request.base.ref }}
-          git diff --name-only --diff-filter=AM origin/${{ github.event.pull_request.base.ref }} HEAD | grep "^servers/" > changed-servers.txt || true
+          git diff --name-only --diff-filter=AM origin/${{ github.event.pull_request.base.ref }}...HEAD | grep "^servers/" > changed-servers.txt || true
 
       - name: Build and catalog changed servers
         shell: bash