Fix the printed path of the ".repo" dir after a fresh init.

Apparently, manifest.topdir already contains a trailing slash in some
cases, so a simple string concatenation may or not lead to double
slashes. It is safer to use os.path.join. See
https://screenshot.googleplex.com/6pSY3QewAeCdAqk

Change-Id: I2411452296b7e78fc975787b675273a48d6b3d85
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/341574
Reviewed-by: LaMont Jones <lamontjones@google.com>
Tested-by: Mateus Azis <azis@google.com>
diff --git a/subcmds/init.py b/subcmds/init.py
index cced44d..e4df6b3 100644
--- a/subcmds/init.py
+++ b/subcmds/init.py
@@ -241,7 +241,7 @@
     if current_dir != self.manifest.topdir:
       print('If this is not the directory in which you want to initialize '
             'repo, please run:')
-      print('   rm -r %s/.repo' % self.manifest.topdir)
+      print('   rm -r %s' % os.path.join(self.manifest.topdir, '.repo'))
       print('and try again.')
 
   def ValidateOptions(self, opt, args):