object.h: stop depending on cache.h; make cache.h depend on object.h

object.h: stop depending on cache.h; make cache.h depend on object.h

Things should be able to depend on object.h without pulling in all of
cache.h.  Move an enum to allow this.

Note that a couple files previously depended on things brought in
through cache.h indirectly (revision.h -> commit.h -> object.h ->
cache.h).  As such, this change requires making existing dependencies
more explicit in half a dozen files.  The inclusion of strbuf.h in
some headers if of particular note: these headers directly embedded a
strbuf in some new structs, meaning they should have been including
strbuf.h all along but were indirectly getting the necessary
definitions.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>