Improve error handling in git commands within action.yml (#479)

- Added warnings for failed fetch of the head branch if it has been deleted and  ensure the action continues even if a checkout fails.

Co-authored-by: Linearb CI <linearb-ci@gmail.com>

Author: yeelali14

action.yml

@@ -86,10 +86,10 @@ runs:
         git config checkout.defaultRemote origin
         git fetch --shallow-since="6 months ago" origin $'${{ steps.safe-strings.outputs.base_ref }}'
         git remote add upstream $'${{ steps.safe-strings.outputs.url }}'
-        git fetch --shallow-since="6 months ago" upstream $'${{ steps.safe-strings.outputs.head_ref }}'
-        git checkout -b $'upstream/${{ steps.safe-strings.outputs.head_ref}}' $'upstream/${{ steps.safe-strings.outputs.head_ref}}'
-        git checkout $'${{ steps.safe-strings.outputs.base_ref }}'
-        git checkout $'${{ steps.safe-strings.outputs.head_ref }}'
+        git fetch --shallow-since="6 months ago" upstream $'${{ steps.safe-strings.outputs.head_ref }}' || echo "::warning::Failed to fetch head branch. The branch may have been deleted."
+        git checkout -b $'upstream/${{ steps.safe-strings.outputs.head_ref}}' $'upstream/${{ steps.safe-strings.outputs.head_ref}}' || true
+        git checkout $'${{ steps.safe-strings.outputs.base_ref }}' || true
+        git checkout $'${{ steps.safe-strings.outputs.head_ref }}' || true
 
     - name: Create cm folder
       shell: bash