Make the SB mirroring pipeline work for 8.0 preview 1

[premun]

There were a couple of minor things such as it could only push to an already existing branch.

I was able to mirror a dev branch: https://dev.azure.com/dnceng/internal/_build/results?buildId=2112291&view=logs&j=011e1ec8-6569-5e69-4f06-baf193d1351e&t=bf6cf4cf-6432-59cf-d384-6b3bcf32ede2

#3194

[MichaelSimons]

@premun - Can you clarify the intent with this change? Your description leads me to think the pipeline was updated to create the branch if it doesn't exist. I am not spotting the change that fixed this, rather it looks like a better UX was implemented to call out the error.

[premun]

@MichaelSimons the || echo Branch doesn't exist yet will return 0 and not fail

[MichaelSimons]

@MichaelSimons the || echo Branch doesn't exist yet will return 0 and not fail

Oh I get it, the change is hiding the error so that git push will create it. Thanks for helping me understand.