I'm not sure --use-commit-message has any real use cases.

This was introduced a very long time ago in D1385 and appears to support a workflow which no longer exists, so I'm just going to remove it.

arc diff has a lot of code to deal with handling untracked or uncommitted changes in the working copy. The major motivations for arc caring about this are:

• we generally can not run unit tests unless we have a working copy; and
• we often can not run lint on files which aren't present in the working copy; and
• unit test results run in a working copy with a bunch of extra changes may not produce the right answer (for example: they may pass, when the change-as-landed fails, because you have uncommitted code which is required for them to pass); and
• at a not-insignificant rate, users simply forget to add files. In ~2010, after I disabled this prompt because several engineers insisted that no one ever forgets to add files, a user forgot to add a file at Facebook and broke all ads for about a day, costing Facebook a trillion zillion dollars.

Generally, many users don't like the behavior of arc here, and would like to keep a big pile of uncommitted changes loose in their working copies at all times and believe this will never lead to errors. I believe they are, on average, wrong, but am also now more eager to give users enough rope to shoot themselves in the feet than I was in the past.