Mark generated files with custom `generated` git diff driver

[erlend-aasland]

In .gitattributes, add diff=generated to generated files so one conveniently can configure git diff to ignore them, for example by marking them as "binary files" (git config diff.generated.binary true). This should not impact how GitHub renders such diffs.

Suggestion stolen from @zware on Discord 😎

[erlend-aasland]

See also:

• https://git-scm.com/docs/diff-config
• https://git-scm.com/docs/gitattributes

[isidentical]

If someone takes over, here is a prepared list from a year back. Probably it got changed a lot, but might be useful base python/cpython#20314

[erlend-aasland]

See also https://bugs.python.org/issue40729

[zware]

Done in python/cpython#30745.