fix(git): stop stash_show_wants_patch from treating -u as patch mode

-u means --include-untracked for git stash show, not -p (unlike git
log, where -u is a -p synonym). Conflating them routed -u's stat-only
output through compact_diff, which only renders patch content,
producing silently empty output where a stat summary was expected.

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