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

autoload/fugitive.vim

@@ -2071,6 +2071,8 @@ function! s:Expand(rev, ...) abort
     endif
   elseif s:Slash(a:rev) =~# '^\a\a\+://'
     let file = substitute(a:rev, '\\\@<!\%(#\a\|%\x\x\)', '\\&', 'g')
+  elseif a:rev =~# '^:[!#%$]'
+    let file = ':0' . a:rev
   else
     let file = a:rev
   endif