Fixing update of dev packages
Closes #961, by reverting the fix to #806 (420401676820894094ef33b22152c5e667453270)
Re-implementing a better fix to #806, delegating the responsability to update the remote URL to
the backend at fetch time. *This is only done for git though*, need to also do it for hg and darcs
handle URL changes in the hg backend

Quickly tested, it should work now.

Darcs shouldn't need any changes.
Merge pull request #962 from AltGr/refs/heads/fix-git-pkg-update

Fixing update of dev packages