[PATCH 0/3] introduce GIT_CACHE_DIRECTORY to plumbing

The first patch introduces the GIT_CACHE_DIRECTORY to the C plumbing. Without this patch, the index file and its lock are always placed in './.git'. Scripts wishing to run these commands from a different working directory can use this support to override the cache directory.

The second patch renames the DB_ENVIRONMENT symbols to match. Note, the name changes came from an off-list comment from Linus, after my sendpatches script accidentally bombed him with a half dozen copies of the init-db patch series summary. His comment inspired these last two.

The third patch renames SHA1_FILE_DIRECTORY as GIT_OBJECT_DIRECTORY, to match the "GIT_CACHE_DIRECTORY" name introduced in the first patch. This was done last and seperately so it can be applied after a little bit of notice. We're not at a point of need to be backwards compatible.

This require my latest init-db cleanups be applied first, otherwise you will get a rejection in init-db.c.

There are 3 patches in this series:
        [PATCH 1/3] add GIT_CACHE_DIRECTORY support
        [PATCH 2/3] rename object directory symbols
        [PATCH 3/3] rename SHA1_FILE_DIRECTORY