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

[sgraenitz]

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

llvm/trunk/utils/git-svn/git-llvm

def svn_push_one_rev(svn_repo, rev, git_to_svn_mapping, dry_run):
     # Split files by subrepo
     subrepo_files = collections.defaultdict(list)
     for f in files:
         subrepo, remainder = split_subrepo(f, git_to_svn_mapping)
         subrepo_files[subrepo].append(remainder)
 
     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):
         svn_sr_path = git_to_svn_mapping[sr]
         for f in files:
             svn_dirs_to_update.add(
                 os.path.dirname(os.path.join(svn_sr_path, f)))