[llvm] [mlir] Update GettingStarted.rst doc with negative refspec to filter user branches (PR #75015)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

Errr, the changes don't match the PR description. Did you want to push something else here?

https://github.com/llvm/llvm-project/pull/75015