lib/commit: Fix object sizes metadata for multiple commits

lib/commit: Fix object sizes metadata for multiple commits

The object sizes hash table was only being cleared when the repo was
finalized. That means that performing multiple commits while the repo
was open would reuse all the object sizes metadata for each commit.

Clear the hash table when the sizes metadata is setup and when it's
added to a commit. This still does not fix the issue all the way since
it does nothing to prevent the program from constructing multiple
commits simultaneously. To handle that, the object sizes hash table
should be attached to the MutableTree since that has the commit state.
However, the MutableTree is gone when the commit is actually created.
The hash table would have to be transferred to the root file when
writing the MutableTree. That would be an awkward addition to
OstreeRepoFile, though. Add a FIXME to capture that.