refactor(arg_tokenizer,git): dedupe the digit-run predicate

git.rs's own is_digit_run duplicated the identical predicate already
inlined in arg_tokenizer::tokenize_dialect's own cluster segmentation.
Exposed arg_tokenizer::is_digit_run as the single shared definition and
had git.rs use it instead of keeping a second copy that could drift.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>