Optionally repack the object repository after every pull

Add an option to .stgitrc which causes the object repository to be repacked
after every "stg pull" operation.  Especially on NFS, this improves overall
performance considerably.  The only downside is that the first time it runs
it will take a very long time if the repository hasn't been packed
recently.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>