Add location of SVN staging dir to git-llvm error output

Summary:
In pre-monorepo times the svn staging directory was `.git/svn`. The below error message wasn't mentioning the new name yet.

Example before:
```
Can't push git rev 104cfa289d9 because svn status is not empty:
!     llvm/trunk/include/llvm
```

Example after:
```
Can't push git rev 104cfa289d9 because status in svn staging dir (.git/llvm-upstream-svn) is not empty:
!     llvm/trunk/include/llvm
```

Reviewers: mehdi_amini, jlebar, teemperor

Reviewed By: mehdi_amini

Subscribers: llvm-commits, #llvm

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65038

llvm-svn: 366671