.azurepipelines/templates/pr-gate-steps.yml: Fetch target branch

Add step to fetch target branch so that PR_EVAL can resolve diff

Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

.azurepipelines/templates/pr-gate-steps.yml

@@ -31,6 +31,12 @@ steps:
     echo "##vso[task.setvariable variable=pkgs_to_build]${{ parameters.build_pkgs }}"
     echo "##vso[task.setvariable variable=pkg_count]${{ 1 }}"
 
+# Fetch the target branch so that pr_eval can diff them.
+# Seems like azure pipelines/github changed checkout process in nov 2020.
+- script: git fetch origin $(System.PullRequest.targetBranch)
+  displayName: fetch target branch
+  condition: eq(variables['Build.Reason'], 'PullRequest')
+
 # trim the package list if this is a PR
 - task: CmdLine@1
   displayName: Check if ${{ parameters.build_pkgs }} need testing