Replace vim-gitgutter with vim-signify

vim-gitgutter is buggy. vim-signify might be better, it does allow
configuration of which events trigger a sign column update and this
patch takes advantage of that. Firstly, the `CursorHold` and
`CursorHoldI` events are disabled and replaced with `InsertLeave` and
`TextChanged` events. This makes the sign column updates immediate, and
since vim-signify is asynchronous it's faster too.

autoload/do.vim

@@ -69,7 +69,7 @@ function! do#rename_include_guard(old)
   call setpos('.', l:pos)
 endfunction
 
-" Setup and start a debugging command.
+" Setup and start a debugging command
 function! do#debug(...)
   packadd termdebug
   let l:command = 'TermdebugCommand'
@@ -86,3 +86,13 @@ function! do#last_change()
     norm!``
   endif
 endfunction
+
+" Augment vim-signify update events
+function! do#signify() abort
+  " Disable update on cursor hold
+  autocmd! signify CursorHold,CursorHoldI
+  " Enable updates on leaving insert mode
+  autocmd signify InsertLeave,TextChanged <buffer> call sy#start()
+  " Enable update on text change e.g. undo/redo
+  autocmd signify TextChanged <buffer> call sy#start()
+endfunction