DATA-PRESENT working for exporttree=yes remotes

DATA-PRESENT working for exporttree=yes remotes

Since the annex-tracking-branch is pushed first, git-annex has already
updated the export database when the DATA-PRESENT arrives. Which means
that just using checkPresent is enough to verify that there is some file
on the special remote in the export location for the key.

So, the simplest possible implementation of this happened to work!

(I also tested it with chunked specialremotes, which also works, as long
as the chunk size used is the same as the configured chunk size. In that
case, the lack of a chunk log is not a problem. Doubtful this will ever
make sense to use with a chunked special remote though, that gets pretty
deep into re-implementing git-annex.)

Updated the client side upload tip with a missing step, and reorged for clarity.