[gardening] Python tidies in cmpcodesize.main

[alexwlchan]

What's in this pull request?

Some Python tidies:

• Those .sorts weren't actually doing anything -- you need to call the methods for them to take effect.
• Use zip(), and drop the idx variable. We know that old_files and new_files are the same length -- there's a check on the lines above -- so we don't need to worry about missing elements.

Resolved bug number: (SR-)

None.

---

Before merging this pull request to apple/swift repository:

• Test pull request on Swift continuous integration.

Triggering Swift CI

The swift-ci is triggered by writing a comment on this PR addressed to the GitHub user @swift-ci. Different tests will run depending on the specific comment that you use. The currently available comments are:

Smoke Testing

Platform	Comment
All supported platforms	@swift-ci Please smoke test
All supported platforms	@swift-ci Please smoke test and merge
OS X platform	@swift-ci Please smoke test OS X platform
Linux platform	@swift-ci Please smoke test Linux platform

A smoke test on macOS does the following:

1. Builds the compiler incrementally.
2. Builds the standard library only for macOS. Simulator standard libraries and
   device standard libraries are not built.
3. lldb is not built.
4. The test and validation-test targets are run only for macOS. The optimized
   version of these tests are not run.

A smoke test on Linux does the following:

1. Builds the compiler incrementally.
2. Builds the standard library incrementally.
3. lldb is built incrementally.
4. The swift test and validation-test targets are run. The optimized version of these
   tests are not run.
5. lldb is tested.

Validation Testing

Platform	Comment
All supported platforms	@swift-ci Please test
All supported platforms	@swift-ci Please test and merge
OS X platform	@swift-ci Please test OS X platform
OS X platform	@swift-ci Please benchmark
Linux platform	@swift-ci Please test Linux platform

Lint Testing

Language	Comment
Python	@swift-ci Please Python lint

Note: Only members of the Apple organization can trigger swift-ci.

• Actually call the sort() method.
• Cut out an unnecessary idx variable.

[CodaFi]

@swift-ci please Python lint.

[CodaFi]

CC @eeckstein for review.

[eeckstein]

@swift-ci Please smoke test