Code clean-up to clarify the rule that "git-compat-util.h" must be
the first to be included.

* en/header-cleanup:
  diff.h: remove unnecessary include of object.h
  Remove unnecessary includes of builtin.h
  treewide: replace cache.h with more direct headers, where possible
  replace-object.h: move read_replace_refs declaration from cache.h to here
  object-store.h: move struct object_info from cache.h
  dir.h: refactor to no longer need to include cache.h
  object.h: stop depending on cache.h; make cache.h depend on object.h
  ident.h: move ident-related declarations out of cache.h
  pretty.h: move has_non_ascii() declaration from commit.h
  cache.h: remove dependence on hex.h; make other files include it explicitly
  hex.h: move some hex-related declarations from cache.h
  hash.h: move some oid-related declarations from cache.h
  alloc.h: move ALLOC_GROW() functions from cache.h
  treewide: remove unnecessary cache.h includes in source files
  treewide: remove unnecessary cache.h includes
  treewide: remove unnecessary git-compat-util.h includes in headers
  treewide: ensure one of the appropriate headers is sourced first