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.

vimrc

@@ -68,12 +68,10 @@ let g:ale_linters = {'c': [], 'cpp': []}
 let g:ale_cmake_cmakelint_options =
       \ '-convention/filename,-package/consistency,-package/stdargs'
 
-" git diff in the sign column
-Pack 'airblade/vim-gitgutter'
+" Version control differences in the sign column
+Pack 'mhinz/vim-signify'
 if exists('&signcolumn')
   set signcolumn=yes
-else
-  let g:gitgutter_sign_column_always = 1
 endif
 
 " format.vim - format with text objects