[rush] Azure DevOps pipelines shallow fetch =1 is now default

[kkazala]

Summary

It's not an issue with rush specifically, but it does have an impact on rush change --verify for example, when invoked in a CI pipeline
Newly created Azure DevOps pipelines have shallow fetch with depth=1 enabled by default, so anything that depends on getMergeBase will fail (in CI, PR works)
Do you think it would make sense to detect it and display a warning?

Repro steps

1. Clone https:/microsoft/rush-example (or use your existing rush repo)
2. Create new Azure DevOps pipeline, using ci.yaml
3. Ensure the shallow fetch is enabled (see Azure DevOps pipelines shallow fetch =1 is now default)
   
4. Make a new branch and push few commits
5. see the results
   

Details

getMergeBase invokes git --no-optional-locks merge-base HEAD targetBranch

which, if fetchDepth=1 and current commit is NOT the first one after branching, fails with ##[error]PowerShell exited with code '1'.

Standard questions

This is not a rush issue, but does cause rush to fail in Azure DevOps CI pipelines
Would be great if a warning was displayed instead of an error =)

[iclanton]

Seems like we should throw an error that specifically says that we can't find a merge base and provide some hints to the user about how to resolve that. Perhaps recommending that they run git fetch --deepen if it's a local clone or increase the fetch depth in the config if it's CI.