[Build] Diff from branch point

Sometimes Jenkins posts [spurious reports of new classes being added](https://github.com/apache/spark/pull/2339#issuecomment-56570170). I believe this stems from diffing the patch against `master`, as opposed to against `master...`, which starts from the commit the PR was branched from.

This patch fixes that behavior.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #2512 from nchammas/diff-only-commits-ahead and squashes the following commits:

c065599 [Nicholas Chammas] comment typo fix
a453c67 [Nicholas Chammas] diff from branch point
[SPARK-3659] Set EC2 version to 1.1.0 and update version map

This brings the master branch in sync with branch-1.1

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #2510 from shivaram/spark-ec2-version and squashes the following commits:

bb0dd16 [Shivaram Venkataraman] Set EC2 version to 1.1.0 and update version map
[SPARK-3634] [PySpark] User's module should take precedence over system modules

Python modules added through addPyFile should take precedence over system modules.

This patch put the path for user added module in the front of sys.path (just after '').

Author: Davies Liu <davies.liu@gmail.com>

Closes #2492 from davies/path and squashes the following commits:

4a2af78 [Davies Liu] fix tests
f7ff4da [Davies Liu] ad license header
6b0002f [Davies Liu] add tests
c16c392 [Davies Liu] put addPyFile in front of sys.path
[SPARK-3679] [PySpark] pickle the exact globals of functions

function.func_code.co_names has all the names used in the function, including name of attributes. It will pickle some unnecessary globals if there is a global having the same name with attribute (in co_names).

There is a regression introduced by #2144, revert part of changes in that PR.

cc JoshRosen

Author: Davies Liu <davies.liu@gmail.com>

Closes #2522 from davies/globals and squashes the following commits:

dfbccf5 [Davies Liu] fix bug while pickle globals of function