Our merge script now turns
```
[SPARK-1234][SPARK-1235][SPARK-1236][SQL] description
```
into
```
[SPARK-1234] [SPARK-1235] [SPARK-1236] [SQL] description
```
The extra spaces are more annoying in git since the first line of a git commit is supposed to be very short.

Doctest passes with the following command:
```
python -m doctest merge_spark_pr.py
```

Author: Reynold Xin <rxin@databricks.com>

Closes #9156 from rxin/SPARK-11169.