client.py,git.py: run 'bup midx -a' automatically sometimes.

client.py,git.py: run 'bup midx -a' automatically sometimes.

Now that 'bup midx -a' is smarter, we should run it automatically after
creating a new index file.  This should remove the need for running it by
hand.

Thus, we also remove 'bup midx' from the lists of commonly-used subcommands.
(While we're here, let's take out 'split' and 'join' too; you should be
using 'index' and 'save' most of the time.)

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>