Merge branch 'rj/header-check'

Header files clean-up.

* rj/header-check:
delta-islands.h: add missing forward declarations (hdr-check)
midx.h: add missing forward declarations (hdr-check)
refs/refs-internal.h: add missing declarations (hdr-check)
refs/packed-backend.h: add missing declaration (hdr-check)
refs/ref-cache.h: add missing declarations (hdr-check)
ewah/ewok_rlw.h: add missing include (hdr-check)
json-writer.h: add missing include (hdr-check)
Makefile: add a hdr-check target

diff --cc Makefile
Simple merge

diff --cc midx.h
Simple merge

diff --cc refs/refs-internal.h
Simple merge