This is the last memory leak that prevents git-annex from running
in constant space, as far as I can see. I can now run git annex find
dummied up to repeatedly find the same file over and over, on millions
olf files, and memory stays entirely constant.
The list of files had to be retained until the end so it could be deleted.
Also, a list of update-index lines was generated and only then fed into it.
Now everything streams in constant space.
When hashing the files, the entire list of shas was read strictly.
That was entirely unnecessary, since there's a cleanup action run
after they're consumed.
