Use a NULL-and-NUL check to see if we have a prefix and consistently use
C string functions on it instead of storing its length in a member of
struct apply_state. This avoids strlen() calls and simplifies the code.
Signed-off-by: Rene Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>