Fix interpretation of :% when % is absolute

We're trying to support two different Git syntaxes here that were never
meant to be mixed.  We want :/foo/bar to be interpreted as the pathspec
:(top)foo/bar, but we want :% to be interpreted as the % file in the
index, even when % is /foo/bar.

Resolves: https://github.com/tpope/vim-fugitive/issues/2404