docs: Add ASCII graphs to interdiff, squash, and duplicate help

[murataslan1]

Summary

This PR addresses #3687 by adding visual ASCII diagrams to the help text of commands that work with commit graphs.

Changes

Added ASCII graphs to:

jj interdiff

Shows how commits are compared by temporarily rebasing one onto the other's parent:

    D
    |
C   C
|   |
B   A'  (A rebased onto C's parent)
|   |
A   B
 \ /
  root

jj squash

Shows how changes are moved from one commit to its parent:

D          D
|          |
C          C
|    =>    |
B          B'  (contains changes from both B and C)
|
A

jj duplicate

Shows how commits are copied:

         C'
C        |
|   =>   C
B        |
         B

Also added #[command(verbatim_doc_comment)] attribute to preserve the ASCII art formatting in the help output.

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[joyously]

The text describes multiple duplicates, but the graph shows one duplicate and a move or something else. Is this graph correct? If so, improve the text so it matches.

[murataslan1]

Fixed! Updated the graph to show C being duplicated to C' with both sharing the same parent B, and added explanatory text.

[joyously]

A novice is left thinking that A was deleted, and C remains even though empty.

[murataslan1]

Fixed! Updated with Before/After format showing that C is abandoned and B' contains both B's and C's changes. A remains in the graph.

[murataslan1]

Thanks for the feedback @joyously! I've updated the graphs to address your concerns:

duplicate.rs:

• First graph now correctly shows C being duplicated to C' (both sharing parent B), not moved
• Second graph uses Before/After format and clearly shows original C remains unchanged

squash.rs:

• Graph now uses Before/After format
• Clearly shows C is abandoned and B' contains both B's and C's changes
• Added explanatory text: "(C is abandoned after its changes are moved into B)"

Please let me know if the updated graphs are clearer now!

[joyously]

Please let me know if the updated graphs are clearer now!

Much clearer, thanks. (I'm not the one to check for accuracy, though.)