It was there to keep it apart from the one which read in from a file on
disk. This other indexer does not exist anymore, so there is no need for
anything other than git_indexer to refer to it.
While here, rename _add() function to _append() and _finalize() to
_commit(). The former change is cosmetic, while the latter avoids
talking about "finalizing", which OO languages use to mean something
completely different.
When given an ODB from which to read objects, the indexer will attempt
to inject the missing bases at the end of the pack and update the
header and trailer to reflect the new contents.
