Packs getting are activated in current darcs HEAD.

For 2.10, we should ensure that they are correctly detected and no
redundant work is done while getting repositories.

Currently most repositories make darcs output the message "Problem while
getting packed repository, getting it normally now.". (for instance
http://darcs.nomeata.de/screen-message/ )

On the aforementioned repository and a couple of other repos without
patches, darcs now no longer tells about trying to grab packs, which is
good.

On packs-enabled repositories (like the release branch), darcs also
handles packs silently but does use them (if up-to-date, see for
instance `darcs clone --verbose http://darcs.net/releases/branch-2.10/`).