Command injection in counsel-git-checkout-action

counsel-git-checkout-action happily executes command substitutes in
git branch names -- e.g.

$ git clone https://github.com/dyntopia/counsel-branch
$ emacs counsel-branch/test

M-x counsel-git-checkout RET RET
/bin/bash: abcd: command not found
error: pathspec 'remotes/origin/' did not match any file(s) known to git.

counsel.el (counsel-git-checkout-action): Quote branch names

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