cmd/{bloom,midx}: clean up progress messages.

cmd/{bloom,midx}: clean up progress messages.

bloom was printing messages more often than necessary on fast computers,
which could overwhelm the stderr output a bit.  Also change to a percentage
+ number of objects, like midx and save do, rather than just printing the
current file number.

And don't print so many lines of output by default: now if bloom isn't
end up doing anything, it doesn't print any output.  And if it does do
something, it prints only one output line per file.

bloom and midx now both print the name of the directory where they're
creating their output files; if you have multiple directories in
.bup/index-cache, it was a little confusing to see them doing
multiple runs for no apparent reason.

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