Handle quickfix autocmd changing buffer

To keep things fast we turn off gitgutter in the current buffer during
quickfix commands, e.g. :vimgrep, and turn it back on afterwards.

However the user may have a quickfix autocmd which makes a different
buffer the current one, e.g. :cwindow. Previously this caused an error
because gitgutter expected the current buffer to remain current; now we
explicitly enable the original current buffer.

Fixes #904.

plugin/gitgutter.vim

@@ -341,8 +341,18 @@ augroup gitgutter
   autocmd BufFilePre  * call s:on_buffilepre(expand('<abuf>'))
   autocmd BufFilePost * call s:on_buffilepost(expand('<abuf>'))
 
-  autocmd QuickFixCmdPre  *vimgrep* let b:gitgutter_was_enabled = gitgutter#utility#getbufvar(expand('<abuf>'), 'enabled') | GitGutterBufferDisable
-  autocmd QuickFixCmdPost *vimgrep* if b:gitgutter_was_enabled | GitGutterBufferEnable | endif | unlet b:gitgutter_was_enabled
+  autocmd QuickFixCmdPre *vimgrep*
+        \ if gitgutter#utility#getbufvar(expand('<abuf>'), 'enabled') |
+        \   let s:gitgutter_was_enabled = expand('<abuf>') |
+        \ else |
+        \   let s:gitgutter_was_enabled = 0 |
+        \ endif |
+        \ GitGutterBufferDisable
+  autocmd QuickFixCmdPost *vimgrep*
+        \ if s:gitgutter_was_enabled |
+        \   call gitgutter#buffer_enable(s:gitgutter_was_enabled) |
+        \ endif |
+        \ unlet s:gitgutter_was_enabled
 augroup END
 
 " }}}