Improve AppVeyor CI configuration

- Always build all commits in main branches and pull requests.

- For other branches, only build the latest commit;
  skip intermediate ones if multiple are pushed in quick succession.

- Enable skip_branch_with_pr, to avoid redundant branch builds when a
  PR is open.

- Increase clone_depth from 1 to 10 to reduce failures during checkout/init
  phase when multiple commits are pushed rapidly.

Author: vaintroub

appveyor.yml

@@ -1,6 +1,40 @@
 version: build-{build}~branch-{branch}
 
-clone_depth: 1
+skip_branch_with_pr: true
+
+before_build:
+  - ps: |
+      $pullRequestNr = $env:APPVEYOR_PULL_REQUEST_NUMBER
+      $branch = $env:APPVEYOR_REPO_BRANCH
+      $commit = $env:APPVEYOR_REPO_COMMIT
+      $repo = $env:APPVEYOR_REPO_NAME
+      $prRepo = $env:APPVEYOR_PULL_REQUEST_HEAD_REPO_NAME
+
+      $isPrFromFork = $pullRequestNr -and ($repo -ne $prRepo)
+      Write-Host "pullRequestNr=$pullRequestNr, isPRFromFork=$isPrFromFork, branch=$branch, commit=$commit, repo=$repo"
+
+      if ($pullRequestNr -and (-not $isPrFromFork)) {
+        $branch = $env:APPVEYOR_PULL_REQUEST_HEAD_REPO_BRANCH
+        $commit = $env:APPVEYOR_PULL_REQUEST_HEAD_COMMIT
+        Write-Host "real branch name of this pull request is $branch, real commit $commit"
+      }
+
+      if ($isPrFromFork) {
+        Write-Host "pull request from a fork detected,  always build."
+      } elseif ($branch -eq "main" -or $branch -match '^\d+\.\d+$') {
+        Write-Host "main branch detected, always build."
+      } else {
+        Write-Host "Checking if this is the latest commit on branch '$branch'..."
+        $remoteHead = git ls-remote https://github.com/$repo.git refs/heads/$branch |
+                      ForEach-Object { ($_ -split "`t")[0] }
+        Write-Host "latest commit=$remoteHead"
+        if ($remoteHead -ne $commit) {
+          Write-Host "Not the latest commit on branch '$branch' — skipping build."
+          Exit-AppveyorBuild
+        }
+      }
+
+clone_depth: 10
 
 build_script:
   # dump some system info