log: show U for uninteresting

The revision walker reserves `^` to denote an uninteresting commit but
never shows these. Denote this by `U` instead to free up `^` for when we
need (and show) it.

revision.c

@@ -4381,7 +4381,7 @@ const char *get_revision_mark(const struct rev_info *revs, const struct commit *
 	if (commit->object.flags & BOUNDARY)
 		return "-";
 	else if (commit->object.flags & UNINTERESTING)
-		return "^";
+		return "U";
 	else if (commit->object.flags & PATCHSAME)
 		return "=";
 	else if (!revs || revs->left_right) {