refactor(arg_tokenizer): add dashdash_index()/has_dashdash(), dedupe 4 call sites

Finding the `--` boundary token's position was hand-rolled
independently at dotnet_cmd.rs:713,864 and git.rs:1328,1352. Added
arg_tokenizer::dashdash_index (index into the tokens slice;
tokens[i].source_index recovers the original-args position) and
has_dashdash (presence check), and switched all 4 call sites to them.

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