Set the work_tree to the working_dir

Grit guesses the `working_dir` for each repo it opens; if it can't find
one Repo#working_dir is nil. This change passes Repo#working_dir to Git
command in Git#native via Git's `--work-tree` command-line option.

README.md

@@ -21,6 +21,7 @@ We patched existing grit library to use it inside GitLab
 * Fixes symlinks omission from diff
 * Added Gemfile
 * Ruby 2.0 support
+* Automatically set the `--work-tree=` option for Git
 * and much more small fixes

lib/grit/git.rb

@@ -93,9 +93,9 @@ module Grit
     attr_accessor :git_dir, :bytes_read, :work_tree
-    def initialize(git_dir)
+    def initialize(git_dir, options={})
       self.git_dir    = git_dir
-      self.work_tree  = git_dir.gsub(/\/\.git$/,'')
+      self.work_tree  = options[:work_tree]
       self.bytes_read = 0
     end
@@ -326,6 +326,7 @@ module Grit
       argv = []
       argv << Git.git_binary
       argv << "--git-dir=#{git_dir}" if base
+      argv << "--work-tree=#{work_tree}" if work_tree
       argv << cmd.to_s.tr('_', '-')
       argv.concat(options_to_argv(options))
       argv.concat(args)

lib/grit/repo.rb

@@ -54,7 +54,7 @@ module Grit
         raise NoSuchPathError.new(epath)
       end
-      self.git = Git.new(self.path)
+      self.git = Git.new(self.path, work_tree: self.working_dir)
     end
     # Public: Initialize a git repository (create it on the filesystem). By