templates: use "+" instead of "#" to denote immutable node in ascii graph

Maybe it's personal preference, but the hash sign looks bigger compared to
the normal "o" nodes, and is slanted. This makes immutable commits stand out
too much. I think "+" is closer to the diamond character used in the unicode
template.

cli/src/config/templates.toml

@@ -196,7 +196,7 @@ coalesce(
     ),
     coalesce(
       if(current_working_copy, "@"),
-      if(immutable, "#"),
+      if(immutable, "+"),
       if(conflict, "x"),
       "o",
     )