Get rid of magic string length constants by using skip_prefix() instead
of memcmp() and use xstrfmt() for building a string instead of a
PATH_MAX-sized buffer, snprintf() and xstrdup().
Signed-off-by: Rene Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>