[SPARK-16818] Exchange reuse incorrectly reuses scans over different sets of partitions

## What changes were proposed in this pull request?

This fixes a bug wherethe file scan operator does not take into account partition pruning in its implementation of `sameResult()`. As a result, executions may be incorrect on self-joins over the same base file relation.

The patch here is minimal, but we should reconsider relying on `metadata` for implementing sameResult() in the future, as string representations may not be uniquely identifying.

cc rxin

## How was this patch tested?

Unit tests.

Author: Eric Liang <ekl@databricks.com>

Closes #14425 from ericl/spark-16818.