Document :write in preview window

See #903.

README.mkd

@@ -533,6 +533,8 @@ let g:gitgutter_async = 0
 
 Add `let g:gitgutter_preview_win_floating = 1` to your `~/.vimrc`.  Note that on Vim this prevents you staging (partial) hunks via the preview window.
 
+On Neovim, the preview hunk command will move the cursor into the floating window if it is already open.
+
 
 #### The appearance of a floating/popup window for hunk previews
 

autoload/gitgutter/hunk.vim

@@ -359,6 +359,11 @@ endfunction
 
 
 function! s:preview(hunk_diff)
+  if g:gitgutter_preview_win_floating && exists('*nvim_set_current_win') && s:winid != 0
+    call nvim_set_current_win(s:winid)
+    return
+  endif
+
   let lines = split(a:hunk_diff, '\r\?\n')
   let header = lines[0:4]
   let body = lines[5:]

doc/gitgutter.txt

@@ -201,11 +201,13 @@ Commands for operating on a hunk:~
 :GitGutterUndoHunk      Undo the hunk the cursor is in.
 
                                               *gitgutter-:GitGutterPreviewHunk*
-:GitGutterPreviewHunk   Preview the hunk the cursor is in.
+:GitGutterPreviewHunk   Preview the hunk the cursor is in or, if you are using
+                        floating preview windows in Neovim and the window is
+                        already open, move the cursor into the window.
 
                         To stage part of the hunk, move to the preview window,
-                        delete any lines you do not want to stage, and
-                        |GitGutterStageHunk|.
+                        delete any lines you do not want to stage, and |write|
+                        or |GitGutterStageHunk|.
 
                         To close a non-floating preview window use |:pclose|
                         or |CTRL-W_z| or |CTRL-W_CTRL-Z|; or normal window-