Argument Clinic: add --dry-run and --diff options

[serhiy-storchaka]

Feature or enhancement

Argument Clinic can only rewrite files in place. To see what a change to a clinic block or to the generator itself produces, you have to let it modify the working tree and then look at git diff.

I propose two options, one of the ideas listed in #108643:

• --dry-run -- parse everything as usual, but write nothing and list the files which would be changed.
• --diff -- the same, but write a unified diff of the changes to the standard output, so that it can be reviewed or applied with patch -p0.

Both work with --make, so --dry-run --make --srcdir . tells whether any generated file is stale.

Linked PRs

• gh-155207: Add --dry-run and --diff options to Argument Clinic #155208
• [3.15] gh-155207: Add --dry-run and --diff options to Argument Clinic (GH-155208) #155213
• [3.14] gh-155207: Add --dry-run and --diff options to Argument Clinic (GH-155208) #155214
• [3.13] gh-155207: Add --dry-run and --diff options to Argument Clinic (GH-155208) #155215