fix(git): restore -- boundary awareness in run_diff's wants_stat/wants_compact

run_diff was the one migrated handler still using raw arg == "--stat"
string scans for wants_stat/wants_compact, missed when run_show and
run_log got the same fix earlier -- a file literally named "--stat"
after -- was misdetected as the real --stat flag and routed to the
raw-passthrough branch instead of RTK's stat+compacted-diff path.
Migrated onto arg_tokenizer, sharing log_takes_value like run_show does.

Also documents (arg_tokenizer.rs) a known, deliberately-unfixed
limitation surfaced by review: a single-segment Msbuild-dialect '/'
path (e.g. "/app", "/tmp") is structurally indistinguishable from a
switch name -- confirmed via Docker that real dotnet/MSBuild itself
only resolves this with a filesystem stat() call, which this tokenizer
can't replicate as a pure function.

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