Use three-dot diffs in URL and xref lint workflows

only run on the files actually modified in a PR, not every file touched on main since the branch point

Author: shoumikhin

.github/workflows/_link_check.yml

@@ -18,9 +18,9 @@ jobs:
       script: |
         ./scripts/lint_urls.sh $(
           [ "${{ github.event_name }}" = "pull_request" ] \
-            && git diff --name-only ${{ github.event.pull_request.base.sha }} ${{ github.event.pull_request.head.sha }} \
+            && git diff --name-only ${{ github.event.pull_request.base.sha }}...${{ github.event.pull_request.head.sha }} \
           || [ "${{ github.event_name }}" = "push" ] \
-            && git diff --name-only ${{ github.event.before }} ${{ github.sha }}
+            && git diff --name-only ${{ github.event.before }}...${{ github.sha }}
         )
 
   lint-xrefs:
@@ -35,7 +35,7 @@ jobs:
       script: |
         ./scripts/lint_xrefs.sh $(
           [ "${{ github.event_name }}" = "pull_request" ] \
-            && git diff --name-only ${{ github.event.pull_request.base.sha }} ${{ github.event.pull_request.head.sha }} \
+            && git diff --name-only ${{ github.event.pull_request.base.sha }}...${{ github.event.pull_request.head.sha }} \
           || [ "${{ github.event_name }}" = "push" ] \
-            && git diff --name-only ${{ github.event.before }} ${{ github.sha }}
+            && git diff --name-only ${{ github.event.before }}...${{ github.sha }}
         )