Add style with branch drawing symbols

[lcheylus]

Some X terminals add recently specific drawing symbols to draw Git branches:

• Kitty Add branch drawing symbols to box characters kovidgoyal/kitty#7681
• Ghostty (by Mitchell Hashimoto, first public release for version 1.0 planned for December 2024) FR: Use branch drawing symbols for jj log for supported terminals jj-vcs/jj#4904
• Issue to add them in Wezterm terminal: Interest in custom Branch drawing symbols 🤔 wezterm/wezterm#6328

Screenshot of a Git log graph using these symbols (Flog tool = Vim/Neovim git branch viewer) https://github.com/rbong/vim-flog/blob/master/img/screen-graph.png

I would be a great addition to add a new rendering Style to git-graph using these specific drawing symbols.

[hungvx-dev]

@mlange-42 Any update for this?

[peso]

@hungvx-dev - A PR for this would be welcome. Although the change is not complex, it is also not a trivial change.

Here is a way to get started:

1. Run cargo doc --open and read about the main steps of the algorithm used.
2. Drill down the documentation on rendering to unicode text.
3. Follow the source code link and look for the Characters and Grid structs.
4. Refactor the code so the inner part of the for loop that compute branch lines in a grid is a separate function.
5. Modify this new function to handle the extra cases needed for these new symbols.

Make a draft PR and use that to ask follow up questions. This would also be an opportunity for me to find documentation areas that needs improvement.