%Rpush: Look for variables in the local scope first.

This is intended as a cleaned up version of #2698 & #2552, which partially addresses the issue in #2550.

The issue had two components:
1. The `%Rpush` line magic did not look for variables in the local scope.
2. The `%Rpush` line magic expected variable names to be separated with spaces.

This pull request addresses only the first of these two issues.  I feel that the issues are sufficiently different to deserve separate consideration (and separate pull requests).  In particular, `%Rpull` currently requires variable names to be separated by spaces as well and I think we should ensure that the semantics for `%Rpush` and `%Rpull` remain identical.

I'm going to close #2698 and link to this issue.