Do not read /.git if it is a directory

On my windows under eclipse with EGit there is a directory named .git,
but that gives warning, when read with file_get_contents:
Warning: file_get_contents(/.git) [function.file-get-contents]: failed
to open stream: Permission denied in \includes\GitInfo.php on line 49

Follow up Ieb79c0b4

Change-Id: I7c3ba323460a2fd3739d3912ee6420aa29311f4f