feat: Add source option to diff

It allows you to easily integrate external source for diff count.
If user has some other plugin keeping track of diff counts. There's
no reason for lualine to make same calculations again . It can just
reuse them. But lualine knows nothing about how other plugin exposes
their api so user will have to glue them together with this option.

README.md

@@ -346,6 +346,10 @@ sections = {
       color_modified = nil, -- changes diff's modified foreground color
       color_removed = nil, -- changes diff's removed foreground color
       symbols = {added = '+', modified = '~', removed = '-'} -- changes diff symbols
+      source = nil, -- A function that works as a data source for diff.
+                    -- it must return a table like
+                    {added = add_count, modified = modified_count, removed = removed_count }
+                    -- Or nil on failure. Count <= 0 won't be displayed.
     }
   }
 }

doc/lualine.txt

@@ -276,6 +276,10 @@ Component specific local options~
         color_modified = nil, -- changes diff's modified foreground color
         color_removed = nil, -- changes diff's removed foreground color
         symbols = {added = '+', modified = '~', removed = '-'} -- changes diff symbols
+        source = nil, -- A function that works as a data source for diff.
+                      -- it must return a table like
+                      {added = add_count, modified = modified_count, removed = removed_count }
+                      -- Or nil on failure. Count <= 0 won't be displayed.
       }
     }
   }

lua/lualine/components/diff.lua

@@ -62,16 +62,23 @@ Diff.new = function(self, options, child)
     }
   end
 
-  vim.api.nvim_exec([[
+  if type(new_instance.options.source) ~= 'function' then
+    -- setup internal source
+    vim.cmd [[
       autocmd lualine BufEnter     * lua require'lualine.components.diff'.update_git_diff_getter()
       autocmd lualine BufWritePost * lua require'lualine.components.diff'.update_git_diff()
-  ]], false)
+    ]]
+  end
 
   return new_instance
 end
 
 -- Function that runs everytime statusline is updated
 Diff.update_status = function(self)
+  if self.options.source then
+    Diff.git_diff = self.options.source()
+  end
+
   if Diff.git_diff == nil then return '' end
 
   local colors = {}