log: clean unneeded objects during `log --remerge-diff`

log: clean unneeded objects during `log --remerge-diff`

The --remerge-diff option will need to create new blobs and trees
representing the "automatic merge" state.  If one is traversing a
long project history, one can easily get hundreds of thousands of
loose objects generated during `log --remerge-diff`.  However, none of
those loose objects are needed after we have completed our diff
operation; they can be summarily deleted.

Add a new helper function to tmp_objdir to discard all the contained
objects, and call it after each merge is handled.

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