Use virtualedit to simplify inserting buffer edits

Previously we used a clumsy hack to handle inserting buffer changes from
the server at the end of a line. However there were cases where this
still wasn't working correctly.

This change temporarily sets virtualedit=all for the buffer, which
simplifies the code and removes the hack.

Author: nickspoons

autoload/OmniSharp/buffer.vim

@@ -100,7 +100,8 @@ function! OmniSharp#buffer#Update(responseBody) abort
   let changes = get(a:responseBody, 'Changes', [])
   if type(changes) == type(v:null) | let changes = [] | endif
 
-  let virtualedit = &virtualedit ==? 'all'
+  let virtualedit_bak = &virtualedit
+  let &l:virtualedit = 'all'
   if len(changes)
     for change in changes
       let text = join(split(change.NewText, '\r\?\n', 1), "\n")
@@ -111,15 +112,6 @@ function! OmniSharp#buffer#Update(responseBody) abort
       let start = [change.StartLine, startCol]
       let end = [change.EndLine, endCol]
       call cursor(start)
-      if startCol > len(getline('.')) && !virtualedit
-        " We can't set a mark after the last character of the line, so add an
-        " extra charaqcter which will be immediately deleted again
-        noautocmd normal! a<
-        if start == end
-          let endCol += 1
-          let end[0] = endCol
-        endif
-      endif
       call cursor(change.EndLine, max([1, endCol - 1]))
       let lineLen = len(getline('.'))
       if change.StartLine < change.EndLine && (endCol == 1 || lineLen == 0)
@@ -156,6 +148,7 @@ function! OmniSharp#buffer#Update(responseBody) abort
     let pos[1] = min([pos[1], line('$')])
     call setpos('.', pos)
   endif
+  let &l:virtualedit = virtualedit_bak
 endfunction
 
 function! OmniSharp#buffer#Valid(...) abort