Meta/Reintegrate: guard for misspelt topic name

Earlier the script just kept going, pretending that the failure from
the "merge" was due to a conflicted change that rerere could take
care of.

Reintegrate

@@ -139,9 +139,11 @@ no)
 		"" | "#"*)
 			echo >&2 "* $branch"
 
-			save=$(git rev-parse --verify HEAD)
+			save=$(git rev-parse --verify HEAD) &&
-			tip=$(git rev-parse --verify "$branch^0")
+			tip=$(git rev-parse --verify "$branch^0") &&
-			mb=$(git merge-base "$tip" "$save")
+			mb=$(git merge-base "$tip" "$save") ||
+			exit
+
 			test "$mb" = "$tip" && continue
 
 			mark_cut