$GIT_DIR returned by get_git_dir() is normalized, with all symlinks
resolved (see setup_work_tree function). In order to match paths (or
patterns) against $GIT_DIR char-by-char, they have to be normalized
too. There is a note in config.txt about this, that the user need to
resolve symlinks by themselves if needed.

The problem is, we allow certain path expansion, '~/' and './', for
convenience and can't ask the user to resolve symlinks in these
expansions. Make sure the expanded paths have all symlinks resolved.

PS. The strbuf_realpath(&text, get_git_dir(), 1) is still needed because
get_git_dir() may return relative path.
Noticed-by: NTorsten Bögershausen <tboegi@web.de>
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>