flag: exit 0 when -h or -help invoked but undefined

[sding3]

flag treats -h or -help as a special case to print a nice help
message, but exit with a status code of 2. This update makes
that status code 0.

Fixes #37533

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Hi Ian, it appears that the proposal was accepted earlier today @ #37533 . So I'm curious if we could move forward on this CL. Thanks.

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This change should have a test.

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This change should have a test.

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Thanks for the review. I've added a test, updated the html doc, and pushed updates to address the 4 comments.

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