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

Author: weliveindetail

llvm/utils/git-svn/git-llvm

@@ -312,8 +312,8 @@ def svn_push_one_rev(svn_repo, rev, git_to_svn_mapping, dry_run):
 
     status = svn(svn_repo, 'status', '--no-ignore')
     if status:
-        die("Can't push git rev %s because svn status is not empty:\n%s" %
-            (rev, status))
+        die("Can't push git rev %s because status in svn staging dir (%s) is "
+            "not empty:\n%s" % (rev, svn_repo, status))
 
     svn_dirs_to_update = set()
     for sr, files in iteritems(subrepo_files):