magit-unambiguous-refname-p: fix inverted return value

This fixes 34281ba7785d26e6113f6190868cd3c86404093d which attempted
replaced `magit-AMBIGUOUS-refname-p' with `magit-UNambiguous-refname-p'.
However it only changed the function name and documentation, not what
it actually does.
magit-font-lock-keywords: fix for Emacs version 24.4
magit-{popup,tests}-font-lock-keywords: new options

Seed them with keywords previously defined in `magit-font-lock-keywords'.