subrepo: setting LC_MESSAGES only works if LC_ALL is empty or unset

For example LC_ALL=de_DE.utf-8 would cause the version check to fail,
because "svn, Version 1.6.12 (r955767)" with a capital "V" will be printed.
Using "svn --version --quiet" would only print the version number, but then
matching other messages, e.g. "Committed revision" would fail.

mercurial/subrepo.py

@@ -655,6 +655,10 b' class svnsubrepo(abstractsubrepo):'
             cmd.append(path)
         env = dict(os.environ)
         # Avoid localized output, preserve current locale for everything else.
+        lc_all = env.get('LC_ALL')
+        if lc_all:
+            env['LANG'] = lc_all
+            del env['LC_ALL']
         env['LC_MESSAGES'] = 'C'
         p = subprocess.Popen(cmd, bufsize=-1, close_fds=util.closefds,
                              stdout=subprocess.PIPE, stderr=subprocess.PIPE,