improve detecting whether we are currently in a git repo

lib/hub/context.rb

@@ -123,14 +123,6 @@ module Hub
       GIT_CONFIG["config alias.#{name}"]
     end
 
-    # Core.repositoryformatversion should exist for all git
-    # repositories, and be blank for all non-git repositories. If
-    # there's a better config setting to check here, this can be
-    # changed without breaking anything.
-    def is_repo?
-      GIT_CONFIG['config core.repositoryformatversion']
-    end
-
     def github_url(options = {})
       repo = options[:repo]
       user, repo = repo.split('/') if repo && repo.index('/')
@@ -169,6 +161,14 @@ module Hub
       DIRNAME
     end
 
+    def git_dir
+      GIT_CONFIG['rev-parse --git-dir']
+    end
+
+    def is_repo?
+      !!git_dir
+    end
+
     # Cross-platform web browser command; respects the value set in $BROWSER.
     # 
     # Returns an array, e.g.: ['open']

test/hub_test.rb

@@ -46,7 +46,7 @@ class HubTest < Test::Unit::TestCase
       'config branch.master.remote'  => 'origin',
       'config branch.master.merge'   => 'refs/heads/master',
       'config --bool hub.http-clone' => 'false',
-      'config core.repositoryformatversion' => '0'
+      'rev-parse --git-dir' => '.git'
     )
     super
   end