apply: remove prefix_length member from apply_state

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>

apply.h

@@ -35,7 +35,6 @@ enum apply_verbosity {
 
 struct apply_state {
 	const char *prefix;
-	int prefix_length;
 
 	/* These are lock_file related */
 	struct lock_file *lock_file;