Do thumb attempt-failures limitting via sha1 instead of name.

Otherwise if there is a broken file, and somebody fixes the file,
it could take an hour before thumbs are generated again. This
might confuse the user, and they may revert back to the broken
version thinking their fix didn't work.

Originally reported by Fae on wiki.

Change-Id: I024702391e5f7ccf10a406a29485601f64069405
Merge "Do thumb attempt-failures limitting via sha1 instead of name."