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

This allows to keep fetching release branches as well.

Author: joker-eph

llvm/docs/GettingStarted.rst

@@ -42,10 +42,14 @@ Getting the Source Code and Building LLVM
 
      ``git clone --depth 1 https://github.com/llvm/llvm-project.git``
 
-   * You are likely only interested in the main branch moving forward, if
-     you don't want `git fetch` (or `git pull`) to download user branches, use:
+   * You are likely not interested in the user branches in the repo (used for
+     stacked pull-requests and reverts), you can filter them from your
+     `git fetch` (or `git pull`) with this configuration:
 
-     ``sed 's#fetch = +refs/heads/\*:refs/remotes/origin/\*#fetch = +refs/heads/main:refs/remotes/origin/main#' -i llvm-project/.git/config``
+.. code-block:: console
+
+  git config --add remote.origin.fetch '^refs/heads/users/*'
+  git config --add remote.origin.fetch '^refs/heads/revert-*'
 
 #. Configure and build LLVM and Clang: