fix(): do not run fs_realpath on nil dir

Author: mjlbach

lua/gitsigns/git.lua

@@ -240,7 +240,7 @@ M.get_repo_info = function(path: string, cmd: string): string,string,string
 
   local toplevel = results[1]
   local gitdir = results[2]
-  if not has_abs_gd then
+  if gitdir and not has_abs_gd then
     gitdir = uv.fs_realpath(gitdir)
   end
   local abbrev_head = process_abbrev_head(gitdir, results[3], path, cmd)