Support --black-sentinels command-line option

[edreamleo]

See PR #2985.

• Add the --black-sentinels command-line option.
• Improve support for blackened sentinels, as a form of defensive programming.

Rationale

Leo cannot prevent Leonistas from blackening external files unintentionally.
Black's existence imposes a burden on Leo.

Leo should not prevent Leonistas from blackening external files intentionally.
--black-sentinels avoids huge diffs in projects mandating Black.

Any change to Leo's read/write code is inherently dangerous, but doing nothing may be more dangerous in the long run.

Note: Leo will continue to beautify code using orange, not black.

[tbpassin]

If Leo is to patch Black, it would be important to know that the patching works as intended. IOW, to detect if Black's code has changed in an incompatible way. The easiest way I can think of is to have Black blacken a small snippet - I suppose it would be a sentinel line - and check to make sure it correctly leaves it alone.

The check could be made once per invocation within Leo, and once per command-line invocation. That shouldn't slow down blackening noticeably.

[edreamleo]

it would be important to know that the patching works as intended.

I agree. This issue is speculative at present.

[edreamleo]

Closed via PR #2985. Note: Patching black is out of the question.