Add file_sync_test and start to re-home common files

[jwnimmer-tri]

The idea here is that users should always be able to copy just one subdirectory and end up with everything they need. To the extent that we have important files outside of the drake_... subdirectories, the examples will be broken.

Of course that often will mean duplicating content across the sub-directories, which leads to a maintenance hazard. We can solve that by using this new test program to cross-check that the copies are in sync.

This just moves one file as a starter pack, to bootstrap the idea. Over time, we should make further adjustments and copies so that everything inside the drake_... subdirs is 100% standalone.

For now, maintainers will need to run this by hand. Soon we plan to have #308 to add it to CI.

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This change is 

[jwnimmer-tri]

+@BetsyMcPhail for feature review or delegation, please.

[BetsyMcPhail]

Testing locally on my system, I confirmed that changing a .clang-format file results in an error message.

Reviewed 9 of 9 files at r1, all commit messages.
Reviewable status: 1 unresolved discussion, LGTM missing from assignee betsymcphail, platform LGTM missing (waiting on @jwnimmer-tri)

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private/test/file_sync_test.py line 37 at r1 (raw file):

    # For readability, enforce that the lists are sorted.
    first_name = Path(paths[0]).name
    prologue = f"The {index+1}th list of files (containing {first_name})"

Nit: As there is only one file copied, this results in the error message "ERROR: The 1th list of files (containing .clang-format) do not all match"

[BetsyMcPhail]

Reviewable status: 1 unresolved discussion, platform LGTM from [betsymcphail] (waiting on @jwnimmer-tri)

[jwnimmer-tri]

+@rpoyner-tri for platform review, please.

Reviewable status: all discussions resolved, LGTM missing from assignee rpoyner-tri, platform LGTM from [betsymcphail] (waiting on @rpoyner-tri)

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private/test/file_sync_test.py line 37 at r1 (raw file):

Previously, BetsyMcPhail (Betsy McPhail) wrote…

Nit: As there is only one file copied, this results in the error message "ERROR: The 1th list of files (containing .clang-format) do not all match"

Right. In the future we will have many more groups of copied files, and I don't know of a good way to have the f-string use proper grammar easily.

[BetsyMcPhail]

Reviewable status: all discussions resolved, LGTM missing from assignee rpoyner-tri, platform LGTM from [betsymcphail] (waiting on @rpoyner-tri)

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private/test/file_sync_test.py line 37 at r1 (raw file):

Previously, jwnimmer-tri (Jeremy Nimmer) wrote…

Right. In the future we will have many more groups of copied files, and I don't know of a good way to have the f-string use proper grammar easily.

Works for me, the important part is the failure.

[rpoyner-tri]

Reviewed 9 of 9 files at r1, all commit messages.
Reviewable status: all discussions resolved, LGTM missing from assignee rpoyner-tri, platform LGTM from [betsymcphail] (waiting on @jwnimmer-tri)

[rpoyner-tri]

Reviewable status: complete! all discussions resolved, platform LGTM from [betsymcphail, rpoyner-tri] (waiting on @jwnimmer-tri)

[rpoyner-tri]

Reviewable status: 1 unresolved discussion, platform LGTM from [betsymcphail, rpoyner-tri] (waiting on @jwnimmer-tri)

a discussion (no related file):
Huh. Merge button is there for me, but doesn't work.

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