[3.6] Add short-circuit for doc changes to AppVeyor (GH-5628)

(cherry picked from commit 28607e0dd9)

.github/appveyor.yml

@@ -7,6 +7,22 @@ branches:
     - buildbot-custom
 cache:
   - externals -> PCbuild\*
+before_build:
+  - ps: |+
+      if ($env:APPVEYOR_RE_BUILD) {
+        echo 'Doing full build due to re-build request.'
+      } elseif (!$env:APPVEYOR_PULL_REQUEST_HEAD_COMMIT) {
+        echo 'Not a PR, doing full build.'
+      } else {
+        $mergebase = git merge-base $env:APPVEYOR_PULL_REQUEST_HEAD_COMMIT $env:APPVEYOR_REPO_BRANCH
+        $changes = git diff --name-only HEAD $mergebase | grep -vE '(\.rst$)|(^Doc)|(^Misc)'
+        If (!$changes) {
+          echo 'Only docs were updated, stopping build process.'
+          Exit-AppveyorBuild
+        }
+      }
+
+
 build_script:
   - cmd: PCbuild\build.bat -e
   - cmd: PCbuild\win32\python.exe -m test.pythoninfo