[MERGE] make 'install_bundles' much cheaper

[Ian Clatworthy]

John Arbash Meinel wrote:
> The attached patch just does one simple thing. If we have already
> validated all we can with this bundle, don't do it again.
>
> It drops the "install_bundle" times drastically, and without loss of
> correctness.
>
>
> bzr selftest --benchmark Install.\*few_files_small_tree_100_revision
>
> test_few_files_small_tree_100_revision   OK     1354ms/    5720ms
> test_few_files_small_tree_100_revision   OK    38071ms/   42625ms
>
>   
Excellent - just the sort of thing we need to weed out!

If it's not too late and the risk is low, I'd like to suggest we put 
this in 0.16.

Ian C.