refactor(git): centralize restore_double_dash at the run() dispatcher

restore_double_dash was called ad hoc inside 6 of 13 git subcommand
handlers (run_diff, run_show, run_log, run_add, run_checkout,
run_branch), while run_status, run_commit, run_push, run_pull,
run_fetch, run_stash, and run_worktree got none -- any future
boundary-sensitive logic added to one of those seven would silently
operate on clap's --stripped args unless its author remembered this
per-function convention, the exact class of omission issue #1215 was
about. restore_double_dash is a no-op when -- wasn't stripped, so
calling it once in the dispatcher is safe and free for every handler
that doesn't need it, and closes the gap for ones that will.

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