hunks: add support for gitsigns.nvim

Refactor s:get_hunks_coc() so that parsing the hunk status can be reused
by s:get_hunks_gitsigns().

Reorder s:get_hunks_coc() for consistent ordering with related code.

Give gitsigns.nvim priority over coc-git when selecting b:source_func in
hunks#get_raw_hunks().

autoload/airline/extensions.vim

@@ -234,6 +234,7 @@ function! airline#extensions#load()
         \ || exists('g:loaded_gitgutter')
         \ || exists('g:loaded_changes')
         \ || exists('g:loaded_quickfixsigns')
+        \ || exists(':Gitsigns')
         \ || exists(':CocCommand'))
     call airline#extensions#hunks#init(s:ext)
     call add(s:loaded_ext, 'hunks')