Handle case where .git is not a directory

In some case, `.git` is not a directory but a file (when using submodule). Without this correction, `git-gc-cron` was failing with this stack:

    /Library/Ruby/Gems/1.8/gems/git-gc-cron-0.0.3/lib/git_gc_cron/cli.rb:9:in `chdir': Not a directory - xxxx (Errno::ENOTDIR)
	from /Library/Ruby/Gems/1.8/gems/git-gc-cron-0.0.3/lib/git_gc_cron/cli.rb:9:in `start'
	from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `each_with_index'
	from /Library/Ruby/Gems/1.8/gems/git-gc-cron-0.0.3/lib/git_gc_cron/cli.rb:8:in `each'
	from /Library/Ruby/Gems/1.8/gems/git-gc-cron-0.0.3/lib/git_gc_cron/cli.rb:8:in `each_with_index'
	from /Library/Ruby/Gems/1.8/gems/git-gc-cron-0.0.3/lib/git_gc_cron/cli.rb:8:in `start'
	from /Library/Ruby/Gems/1.8/gems/git-gc-cron-0.0.3/lib/git_gc_cron/cli.rb:6:in `each'
	from /Library/Ruby/Gems/1.8/gems/git-gc-cron-0.0.3/lib/git_gc_cron/cli.rb:6:in `start'
	from /Library/Ruby/Gems/1.8/gems/git-gc-cron-0.0.3/bin/git-gc-cron:4
	from /usr/bin/git-gc-cron:19:in `load'
	from /usr/bin/git-gc-cron:19