[git-llvm] Do not reinvent `@{upstream}` (take 2)
[git-llvm] Do not reinvent `@{upstream}` (take 2)

This makes git-llvm more of a thin wrapper around git while temporarily
maintaining backwards compatibility with past git-llvm behavior.

Using @{upstream} makes git-llvm more robust when used with a nontrivial
local repository.

https://reviews.llvm.org/D67389

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@372070 91177308-0d34-0410-b5e6-96231b3b80d8

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