Convert the parameter subdirname of search_for_subdir() to a
length-limted string and then simply pass the interesting slice of the
refname from find_containing_dir(), thereby avoiding to duplicate the
string.
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx> Signed-off-by: Junio C Hamano <gitster@pobox.com>