zsh -- Arbitrary command execution vulnerability

Marc Cornellà reports:

    Some prompt expansion sequences, such as %F, support 'arguments'
    which are themselves expanded in case they contain colour values,
    etc. This additional expansion would trigger PROMPT_SUBST evaluation,
    if enabled. This could be abused to execute code the user didn't
    expect. e.g., given a certain prompt configuration, an attacker
    could trick a user into executing arbitrary code by having them
    check out a Git branch with a specially crafted name.