[CI] Checkout the head ref rather than the merge or a reset, this should fix issues with the test selector. (#14496)

tests/xharness/GitHub.cs

@@ -164,8 +164,8 @@ namespace Xharness {
 
 		IEnumerable<string> GetModifiedFilesLocally (int pullRequest)
 		{
-			var base_commit = $"origin/pr/{pullRequest}/merge^";
-			var head_commit = $"origin/pr/{pullRequest}/merge";
+			var base_commit = $"origin/pull/{pullRequest}/merge^";
+			var head_commit = $"origin/pull/{pullRequest}/merge";
 
 			harness.Log ("Fetching modified files for commit range {0}..{1}", base_commit, head_commit);
 

tools/devops/automation/templates/build/build.yml

@@ -52,7 +52,13 @@ steps:
 
 - ${{ if eq(variables['Build.Reason'], 'PullRequest') }}:
   - pwsh: |
-      git reset --hard HEAD^2 
+      git config remote.origin.fetch '+refs/pull/*:refs/remotes/origin/pull/*'
+      git fetch origin
+      $branch="$(Build.SourceBranch)".Replace("merge", "head")
+      $branch=$branch.Replace("refs", "origin")
+      Write-Host "Checking out branch $branch"
+      git checkout $branch
+      git branch -a
     displayName: "Undo Github merge"
     workingDirectory: $(System.DefaultWorkingDirectory)/xamarin-macios
 

tools/devops/automation/templates/tests/build.yml

@@ -54,7 +54,13 @@ steps:
 
 - ${{ if eq(variables['Build.Reason'], 'PullRequest') }}:
   - pwsh: |
-      git reset --hard HEAD^2 
+      git config remote.origin.fetch '+refs/pull/*:refs/remotes/origin/pull/*'
+      git fetch origin
+      $branch="$(Build.SourceBranch)".Replace("merge", "head")
+      $branch=$branch.Replace("refs", "origin")
+      Write-Host "Checking out branch $branch"
+      git checkout $branch
+      git branch -a
     displayName: "Undo Github merge"
     workingDirectory: $(System.DefaultWorkingDirectory)/xamarin-macios