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

@@ -4365,7 +4365,7 @@ 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) {