Revert [git-llvm] Do not reinvent `@{upstream}`
Revert [git-llvm] Do not reinvent `@{upstream}`

This reverts r371290 (git commit 7faffd544b16f851a632d6b8f93e3c8485ff34bb)

The change wasnt NFC and broke some users' workflow. Reverting while figuring
out the best alternative to move forward.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@371480 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm