Backport branch mirror pipeline (#41760)

.azure/pipelines/azure-pipelines-mirror-within-azdo.yml

+# Configure which branches trigger builds
+trigger:
+  batch: true
+  branches:
+    include:
+    - internal/release/6.0
+    
+parameters:
+# Run the pipeline manually (usually disallowed)
+- name: manualRun
+  default: false
+  displayName: Are you sure you want to run this pipeline manually?
+  type: boolean
+
+variables:
+  - group: Mirror-Credentials
+
+# Merges code from one AzDO branch into another
+jobs:
+- ${{ if and(contains(variables['Build.SourceBranch'], 'internal'), or(eq(variables['Build.Reason'], 'BatchedCI'), eq(parameters.manualRun, 'true'))) }}:
+  - template: /eng/common/templates/jobs/jobs.yml
+    parameters:
+      enableTelemetry: true
+      helixRepo: dotnet/aspnetcore
+      jobs:
+      - job: Merge_Azure_DevOps_Branches
+        enableSBOM: false
+        pool:
+          name: NetCore1ESPool-Internal
+          demands: ImageOverride -equals Build.Server.Amd64.VS2019
+        variables:
+        - name: WorkingDirectoryName
+          value: repo-dir
+        - name: AzdoRepo
+          value: dotnet-aspnetcore
+        - name: TargetBranchName
+          value: $(Build.SourceBranch)-nonstable
+        - name: BranchToMirror
+          value: $(Build.SourceBranch)
+        steps:
+        - script: |
+            git clone https://dn-bot:$(dn-bot-dnceng-build-rw-code-rw)@dev.azure.com/dnceng/internal/_git/$(AzdoRepo) $(WorkingDirectoryName) --recursive --no-tags --branch $(TargetBranchName)
+          displayName: Clone AzDO repo
+        - script: |
+            git -c user.email="dotnet-bot@microsoft.com" -c user.name="dotnet-bot" merge origin/$(BranchToMirror) -m "Merge in '$(BranchToMirror)' changes"
+          displayName: Merge head branch to target branch
+          workingDirectory: $(WorkingDirectoryName)
+        - script: |
+            git push origin $(TargetBranchName)
+          displayName: Push changes to Azure DevOps repo
+          workingDirectory: $(WorkingDirectoryName)
+
+        - task: PowerShell@1
+          displayName: Broadcast target, branch, commit in metadata
+          continueOnError: true
+          condition: always()
+          inputs:
+            scriptType: inlineScript
+            arguments: '$(BranchToMirror)'
+            workingDirectory: $(WorkingDirectoryName)
+            inlineScript: |
+              param([string]$branch)
+
+              $commit = (git rev-parse HEAD).Substring(0, 7)
+              $target = "$branch".Replace('/', ' ')
+
+              Write-Host "##vso[build.updatebuildnumber]$target $commit"
+              Write-Host "##vso[build.addbuildtag]$target"
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