update proxied exporttree=yes remote on receive of sync branch

update proxied exporttree=yes remote on receive of sync branch

Since git-annex sync sends the sync branch first, and only displays the
output of the push to the sync branch, this makes git-annex
post-retrieve's output when updating the exported tree be visible when
syncing.

This also makes syncing with a non-bare repository still update the
exported tree, even when the checked out branch is not able to be
updated. The sync branch gets sent regardless.