Fixed a bug that could leave a direct mode repository stuck at
annex.version 3. As part of that, v3 indirect mode repositories will be
automatically updated to v5. There's no actual change in that upgrade, it
just simplifies things to have only one supported annex.version.

Added youtube playlist support to git-annex. Seems I had almost all the
pieces needed, and didn't know it. Only about a dozen lines of code!

Added PREPARE-FAILURE support to the external special remote interface.

After I found the cable my kitten stole (her apport level is high), fixed
file transfers to/from Android. This broke because git-annex assistant
tries to use ionice, if it's in PATH, and Android's ionice is not suitable.
It could probably include ionice in the busybox build and use that one, but
I wanted a quick fix for this before the upcoming release.
