git: fix PackIdxList keeping deleted files open

git: fix PackIdxList keeping deleted files open

When an midx is deleted underneath bup, usually by itself running
'bup midx --auto', then PackIdxList may keep them open. This can
cause bup to run out of disk space easily since these files can
be fairly big, and can be recreated multiple times in a backup
run.

To fix this, remove any open PackMidx instances from the list and
close them explicitly.

Out of an abundance of caution, also explicitly close the bloom
instance if we have one - the same issue should apply here even if
I couldn't observe it, since the GC isn't guaranteed to clean up
the object immediately.

I remember debugging this issue years ago without coming to any
good conclusion, and it's been mentioned on the mailing list a few
times as well, e.g.
https://groups.google.com/d/msg/bup-list/AqIyv9n9WPE/-Wl2JVh5AQAJ

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>