Add change 26 to instructions

how_to/Change_26.md

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+- k: Print refs
+
+Now that we have refs and a potentially branching commit history, it's a good
+idea to create a visualization tool to see all the mess that we've created.
+
+The visualization tool will draw all refs and all the commits pointed by the refs.
+
+Our command to run the tool will be called `ugit k`, similar to `gitk` (which is
+a graphical visualization tool for Git).
+
+We'll create a new `k` command in *cli.py*. We'll create `iter_refs` which is a
+generator which will iterate on all available refs (it will return HEAD from the
+ugit root directory and everything under *.ugit/refs*). As a first step, let's
+just print all refs when running `k`.