fixup: error on branch component when stray .git files exist

fixes #490

doc/lualine.txt

@@ -347,7 +347,7 @@ in component.
       theme = 'auto',          -- lualine theme
       component_separators = {left = '', right = ''},
       section_separators = {left = '', right = ''},
-      disabled_filetypes = {},  -- filetypes to diable lualine on
+      disabled_filetypes = {},  -- filetypes to disable lualine on
       always_divide_middle = true, -- When true left_sections (a,b,c) can't
                                    -- take over entiee statusline even
                                    -- when none of section x, y, z is present.

lua/lualine/components/branch/git_branch.lua

@@ -78,9 +78,11 @@ function M.find_git_dir(dir_path)
       elseif git_file_stat.type == 'file' then
         -- separate git-dir or submodule is used
         local file = io.open(git_path)
+        if file then
           git_dir = file:read()
-        git_dir = git_dir:match 'gitdir: (.+)$'
+          git_dir = git_dir and git_dir:match 'gitdir: (.+)$'
           file:close()
+        end
         -- submodule / relative file path
         if git_dir and git_dir:sub(1, 1) ~= sep and not git_dir:match '^%a:.*$' then
           git_dir = git_path:match '(.*).git' .. git_dir