Mirror logic that appends newline on patch application.

[thatch]

The important parts are:

• refactor_file adds a newline. it doesn't take it away, because
• refactor_file calls processed_file which does an ast re-parse
• apply_hunks has at least two bugs; one is the reported one about newlines, but it also appears to not obey cookies.

Ensuring that these 3 paths would always obey cookies and see contents that end with a newline seemed the simpler path. Includes a test that edits the (last, only, missing newline) line.

[thatch]

Windows failures are new with this change; smoke-target goes from having a single LF to many CR followed by LF on disk, and I suspect git may be doing something here. We could paper over and have the patch use the same read_python_source function, but I'm a little uneasy about that.

Thoughts about just writing out a temp file with known contents, to avoid git mangling?

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