apply: reorder functions to move image-related things together

apply: reorder functions to move image-related things together

While most of the functions relating to `struct image` are relatively
close to one another, `fuzzy_matchlines()` sits in between those even
though it is rather unrelated.

Reorder functions such that `struct image`-related functions are next to
each other. While at it, move `clear_image()` to the top such that it is
close to the struct definition itself. This makes this lifecycle-related
thing easy to discover.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>