Report progress while updating references

Report progress while updating references

If a fetch or push needs to apply more than a few references
to the local repository it may take more than 0.25 seconds to
process all of the updates.  This is especially true in the DHT
storage system during an initial push of a project with many tags.
The backend database may need to use a transaction to ensure each
tag reference creation is unique, and there may be large delays
caused by these transactions.

Change-Id: Ib11a077adfbd525253e425d327f2e2c2380804c7
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>