refactor(arg_tokenizer): rename tokenize_git to tokenize_with_separate_value

tokenize_git had exactly one caller in the whole codebase (git.rs's own
git_log_tokens()), and zero git-specific code -- it's a thin wrapper fixing
Dialect::Posix while leaving takes_separate_value customizable, reusable by
any POSIX-ish tool with the same short-flag clustering quirk. Naming a
shared, mechanism-generic function after its only current caller was
misleading: either it should generalize (drop the git-specific name) or it
should live in git-specific code. Since the git-specific rationale (the
-n/-M-family rules) was already documented at git.rs's own
log_takes_separate_value, not lost by renaming, generalizing was the smaller
change.

Trimmed the moved function's doc comment down to the general mechanism,
pointing to log_takes_separate_value for the git-specific quirks it
originally listed inline. Pure rename, no behavior change: cargo test --all
passes the same 2765 tests before and after.

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