Update Dependencies automation always force pushes a rewritten commit

[lbussell]

See this PR for example: https://github.com/dotnet/dotnet-docker/pull/5393

It makes one commit with its updates, and then appears to force push changes that include updates from the base branch. See the diff for the force push: https://github.com/dotnet/dotnet-docker/compare/71a84714b0f607a4e1796cee8afda87a3581644c..255c35ffae79ada42267f9bc1d5b00e50f3cee84?diff=unified&w=

This should just be one commit based on top of the latest commit from the base branch.

[dotnet-issue-labeler[bot]]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[lbussell]

[Triage] This also means that the automation doesn't handle updating changes to an existing branch very well - they always get overwritten or the change gets skipped.