Don't remove installed roots on upgrade ...
Consistant naming of dependencies / reverse-dependencies
Try to improve partial upgrade
Fix <git-path>#<revision> scheme when the revision is a tag (it was only working with SHA1 of commits).
Fix #405

We now uninstall all the packages before starting the installation process.

This means that we must be very carefull when error arise, or the user will be let in a state where everything is broken. I think I found some bugs in the previous attempt to recover from errors, so we are now much more conservative (and hopefully safer). The rule now is:
* try to install most of the packages (ie. a package is installed if none of its dependencies have an error)
* on error, try to install the previous version of the package. If it's not possible, do nothing. Ideally, we should run the solver again to find a consistent solution without the eror package but I guess it's a bit overkill.
Merge branch 'master' of github.com:OCamlPro/opam