Prompt user to choose hunk when overlapping.

See #556.

test/test_gitgutter.vim

@@ -455,6 +455,42 @@ function Test_undo_nearby_hunk()
 endfunction
 
 
+function Test_overlapping_hunk_op()
+  func Answer(char)
+    call feedkeys(a:char."\<CR>")
+  endfunc
+
+  " Undo upper
+
+  execute '3d'
+  execute '1d'
+  call s:trigger_gitgutter()
+  normal gg
+  call timer_start(100, {-> Answer('u')} )
+  GitGutterUndoHunk
+  call s:trigger_gitgutter()
+
+  let expected = [
+        \ 'line=2  id=3000  name=GitGutterLineRemoved',
+        \ ]
+  call assert_equal(expected, s:signs('fixture.txt'))
+
+  " Undo lower
+
+  execute '1d'
+  call s:trigger_gitgutter()
+  normal gg
+  call timer_start(100, {-> Answer('l')} )
+  GitGutterUndoHunk
+  call s:trigger_gitgutter()
+
+  let expected = [
+        \ 'line=1  id=3000  name=GitGutterLineRemovedFirstLine',
+        \ ]
+  call assert_equal(expected, s:signs('fixture.txt'))
+endfunction
+
+
 function Test_write_option()
   set nowrite