tap: fix repair not working in some cases

[Bo98]

I recently got:

Error: Some taps failed to update!
The following taps can not read their remote branches:
  heroku/brew
This is happening because the remote branch was renamed or deleted.
Reset taps to point to the correct remote branches by running `brew tap --repair`

but the instructed brew tap --repair didn't work:

$ brew tap --repair
fatal: couldn't find remote ref refs/heads/master
Error: Failure while executing; `git -C /usr/local/Homebrew/Library/Taps/heroku/homebrew-brew fetch origin` exited with 128.

That was because of some previous default:

$ git -C /usr/local/Homebrew/Library/Taps/heroku/homebrew-brew config remote.origin.fetch
+refs/heads/master:refs/remotes/origin/master