Fix updating the registry too often

It turns out that the registry was being queried with git dependencies as well,
so this commit alters the core registry not query sources with dependencies that
did not originate from the source.

Closes #991
auto merge of #1019 : alexcrichton/cargo/issue-991, r=brson

It turns out that the registry was being queried with git dependencies as well,
so this commit alters the core registry not query sources with dependencies that
did not originate from the source.

Closes #991