A careless invocation of "git send-email directory/" after editing
0001-change.patch with an editor often ends up sending both
0001-change.patch and its backup file, 0001-change.patch~, causing
embarrassment and a minor confusion.  Detect such an input and
offer to skip the backup files when sending the patches out.

* jc/send-email-skip-backup:
  send-email: detect and offer to skip backup files