Fix upstream-pending: use git --grep instead of shell parsing

The first version read git log through a shell while-loop with a tab IFS, which make
mangled — every commit came out as an 'ambiguous argument' error. git's own --grep and
--invert-grep search the full commit message, including the trailer, and need no
parsing at all.

Upstream: no — tooling for forks.

Makefile

@@ -46,14 +46,10 @@ check-fork-divergence: ## Verify the fork differs from upstream only under deplo
 		echo "Fork is clean: no differences outside deploy/prod/."; \
 	fi
 
-upstream-pending: ## For a fork: list local commits marked "Upstream: yes" that upstream lacks
+upstream-pending: ## For a fork: list local commits owed upstream
 	@git fetch --quiet upstream 2>/dev/null || { echo "No 'upstream' remote."; exit 1; }
-	@echo "Marked for upstream, not yet there:"
-	@git log --format='%h %s%n%b' upstream/main..HEAD 2>/dev/null \
-	  | awk '/^[0-9a-f]{7,} /{h=$$0} /^Upstream: *[Yy]es/{print "  " h}' \
-	  | grep . || echo "  (none)"
+	@echo "Marked 'Upstream: yes' and not yet upstream:"
+	@git log --format='  %h %s' --grep='^Upstream: *[Yy]es' upstream/main..HEAD | grep . || echo "  (none)"
 	@echo
-	@echo "Unmarked — decide and amend, or ignore:"
-	@git log --format='%H%x09%h %s' upstream/main..HEAD 2>/dev/null | while IFS=$$'\t' read -r sha line; do \
-	  git log -1 --format='%B' "$$sha" | grep -qi '^Upstream:' || echo "  $$line"; \
-	done | grep . || echo "  (none)"
+	@echo "No Upstream: marker — decide, then amend the message:"
+	@git log --format='  %h %s' --invert-grep --grep='^Upstream:' upstream/main..HEAD | grep . || echo "  (none)"