repo: Store transaction file, use it to optimize for pull resumes

repo: Store transaction file, use it to optimize for pull resumes

If pull is interrupted, we may have downloaded an arbitrary subset of
the requested objects.  Previously, we handled this by scanning for
all objects each time.

However, there's an easy optimization - this patch creates a lock file
in the repo.  If we don't see that file when starting a pull, we know
we don't need to stat() every file; presence of a dirtree object for
example implies the existence of everything it references.