lfs: infer the blob store URL from an explicit pull source
I don't see any easier way to do this because the update part of `hg pull -u`
happens outside exchange.pull(), and commands.postincoming() doesn't take a
path. So (ab)use the mechanism used by subrepos to redirect where subrepos are
pulled from when an explicit path is given. As a bonus, this should allow lfs
blobs to be pulled into a subrepo when it is checked out.
An explicit push path can be handled within exchange.push(). That can be done
next, outside of this dirty hack.
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors