fix(backport): checkout from staging branch and ask to update if needed

[rluvaton]

Fixes #731

this aligns with steps 1-3 in doc/contributing/backporting-to-release-lines.md - How to submit a backport pull request

https://github.com/nodejs/node/blob/c2cd74453e7d2794ad81cab63e68371e08bad04f/doc/contributing/backporting-to-release-lines.md?plain=1#L48-L62

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (6d68c99) 83.03% compared to head (e8a3095) 83.03%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #732 +/- ## ======================================= Coverage 83.03% 83.03% ======================================= Files 37 37 Lines 4178 4178 ======================================= Hits 3469 3469 Misses 709 709 ```

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[Trott]

Is it reasonable to add a test for this change? (Non-blocking but of course I'd prefer a test to not having test if it's not too onerous.)

[rluvaton]

I totally with you on adding test but there where no test to begin with

I will try to add one...

[rluvaton]

Please do not hard code 'upstream', and take the name of the upstream from the config instead.

@aduh95 fixed