apply: remove prefix_length member from apply_state

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>