git-dangling: Move all $bannedfile tests to the end

If there's many dangling commits, it's very inefficient to check while
scanning through dangling-result.tmp, instead loop through it at the end
and delete those branches, tags and blobs that should not be restored.

40c16956-1aca-11e4-b1ae-fefdb24f8e10
git-dangling: Use xargs(1) when initially deleting commit-* branches

To avoid the dreaded "Argument list too long" error if there's tons of
commit-* branches.

537dd680-1ac8-11e4-a25d-fefdb24f8e10