Merge branch 'ps/apply-leakfix'

"git apply" had custom buffer management code that predated before
use of strbuf got widespread, which has been updated to use strbuf,
which also plugged some memory leaks.

* ps/apply-leakfix:
  apply: refactor `struct image` to use a `struct strbuf`
  apply: rename members that track line count and allocation length
  apply: refactor code to drop `line_allocated`
  apply: introduce macro and function to init images
  apply: rename functions operating on `struct image`
  apply: reorder functions to move image-related things together

Author: gitster