[SPARK-2176][SQL] Extra unnecessary exchange operator in the result of an explain command

```
hql("explain select * from src group by key").collect().foreach(println)

[ExplainCommand [plan#27:0]]
[ Aggregate false, [key#25], [key#25,value#26]]
[  Exchange (HashPartitioning [key#25:0], 200)]
[   Exchange (HashPartitioning [key#25:0], 200)]
[    Aggregate true, [key#25], [key#25]]
[     HiveTableScan [key#25,value#26], (MetastoreRelation default, src, None), None]
```

There are two exchange operators.

However, if we do not use explain...
```
hql("select * from src group by key")

res4: org.apache.spark.sql.SchemaRDD =
SchemaRDD[8] at RDD at SchemaRDD.scala:100
== Query Plan ==
Aggregate false, [key#8], [key#8,value#9]
 Exchange (HashPartitioning [key#8:0], 200)
  Aggregate true, [key#8], [key#8]
   HiveTableScan [key#8,value#9], (MetastoreRelation default, src, None), None
```
The plan is fine.

The cause of this bug is explained below.

When we create an `execution.ExplainCommand`, we use the `executedPlan` as the child of this `ExplainCommand`. But, this `executedPlan` is prepared for execution again when we generate the `executedPlan` for the `ExplainCommand`. Basically, `prepareForExecution` is called twice on a physical plan. Because after `prepareForExecution` we have already bounded those references (in `BoundReference`s), `AddExchange` cannot figure out we are using the same partitioning (we use `AttributeReference`s to create an `ExchangeOperator` and then those references will be changed to `BoundReference`s after `prepareForExecution` is called). So, an extra `ExchangeOperator` is inserted.

I think in `CommandStrategy`, we should just use the `sparkPlan` (`sparkPlan` is the input of `prepareForExecution`) to initialize the `ExplainCommand` instead of using `executedPlan`.

The link to JIRA: https://issues.apache.org/jira/browse/SPARK-2176

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1116 from yhuai/SPARK-2176 and squashes the following commits:

197c19c [Yin Huai] Use sparkPlan to initialize a Physical Explain Command instead of using executedPlan.

(cherry picked from commit 587d32012ceeec1e80cec1878312f164cdb76ec8)
Signed-off-by: Reynold Xin <rxin@apache.org>
[SPARK-2184][SQL] AddExchange isn't idempotent

...redPartitioning.

Author: Michael Armbrust <michael@databricks.com>

Closes #1122 from marmbrus/fixAddExchange and squashes the following commits:

3417537 [Michael Armbrust] Don't bind partitioning expressions as that breaks comparison with requiredPartitioning.

(cherry picked from commit 5ff75c748a27bcfae71759d0e509218f0c5d0200)
Signed-off-by: Reynold Xin <rxin@apache.org>
Squishing a typo bug before it causes real harm

in updateNumRows method in RowMatrix

Author: Doris Xin <doris.s.xin@gmail.com>

Closes #1125 from dorx/updateNumRows and squashes the following commits:

8564aef [Doris Xin] Squishing a typo bug before it causes real harm

(cherry picked from commit 566f70f2140c1d243fe2368af60ecb390ac8ab3e)
Signed-off-by: Xiangrui Meng <meng@databricks.com>
[SPARK-2187] Explain should not run the optimizer twice.

@yhuai @marmbrus @concretevitamin

Author: Reynold Xin <rxin@apache.org>

Closes #1123 from rxin/explain and squashes the following commits:

def83b0 [Reynold Xin] Update unit tests for explain.
a9d3ba8 [Reynold Xin] [SPARK-2187] Explain should not run the optimizer twice.

(cherry picked from commit 640c294369f49a7602c33c7c389088aec8a316d3)
Signed-off-by: Reynold Xin <rxin@apache.org>