Handle repeated 'g' flags passed to :Acks

This is amazingly edge casey, but the 'g' flag can be repeated, toggling
the effect each time. So:

  :Acks /foo/bar/gggg

Would actually have the effect of turning off global replacement. As
bf9bffb said:

    The docs for `:Acks` promise that:

    > Takes all of the files currently in the |quickfix| listing and
    > performs a substitution of all instances of {pattern} (a standard
    > Vim search |pattern|) by {replacement}.

So in order to be true to the promise of replacing "all instances" we
need to make sure that there is only a single 'g' flag.

autoload/ferret/private.vim

@@ -348,8 +348,14 @@ function! ferret#private#acks(command) abort
   if l:options !~# 'e'
     let l:options.='e'
   endif
-  if !&gdefault && l:options !~# 'g'
-    let l:options.='g'
+  if !&gdefault
+    if l:options !~# 'g'
+      let l:options.='g'
+    else
+      " Make sure there is exactly one 'g' flag present, otherwise an even
+      " number of 'g' flags will actually cancel each other out.
+      let l:options=substitute(l:options, 'g', '', 'g') . 'g'
+    endif
   elseif &gdefault && l:options =~# 'g'
     " 'gdefault' inverts the meaning of the 'g' flag, so we must strip it.
     let l:options=substitute(l:options, 'g', '', 'g')