Be sure to update all packages from a git source

With the recent resolve rewrite, `cargo update -p foo` would only update one
package in a git repository, even if the repository provided many packages.
Cargo does not currently support depending on the same git repository source
with different precise revisions, and this would cause errors down the line
depending on what happened.

This adds a fix to the `resolve_with_previous` method to ensure that any
non-registry sources being updated will not have any locked packages inside them
which would result in an invalid lockfile.
auto merge of #773 : alexcrichton/cargo/update-all-packages-from-a-git-repo, r=brson

With the recent resolve rewrite, `cargo update -p foo` would only update one
package in a git repository, even if the repository provided many packages.
Cargo does not currently support depending on the same git repository source
with different precise revisions, and this would cause errors down the line
depending on what happened.

This adds a fix to the `resolve_with_previous` method to ensure that any
non-registry sources being updated will not have any locked packages inside them
which would result in an invalid lockfile.