apply: rename members that track line count and allocation length

apply: rename members that track line count and allocation length

The `struct image` has two members `nr` and `alloc` that track the
number of lines as well as how large its array is. It is somewhat easy
to confuse these members with `len` though, which tracks the length of
the `buf` member.

Rename these members to `line_nr` and `line_alloc` respectively to avoid
confusion. This is in line with how we typically name variables that
track an array in this way.

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