Kitten difftool crashes on launch

[elliotwesoff]

Describe the bug
Attempting to use the kitty difftool errors and prints a stack trace.

Strangely, the kitty difftool works correctly in most other repositories on my system. The diff on this repository is purely mode changes on the entire repo. I was literally instructed to run chmod -R 755 .

To Reproduce
Steps to reproduce the behavior:

1. Use the following git difftool config in ~/.gitconfig:

[diff]
    tool = kitty
    guitool = kitty.gui
[difftool]
    prompt = false
    trustExitCode = true
[difftool "kitty"]
    cmd = kitty +kitten diff $LOCAL $REMOTE
[difftool "kitty.gui"]
    cmd = kitty kitty +kitten diff $LOCAL $REMOTE

2. Clone the repository: https://github.com/mit-pdos/xv6-riscv
3. Recursively change the file modes in the repo: chmod -R 755 xv6-riscv
4. Run the command git difftool --no-symlinks --dir-diff
5. Be sad because the wonderful kitty difftool doesn't launch
6. Observe the stack trace:

elliot@cavejohnson ~/c/xv6 (riscv)> git difftool --no-symlinks --dir-diff

Paniced with error: runtime error: slice bounds out of range [:-1]
Stacktrace:
goroutine 1 [running]:
runtime/debug.Stack()
	runtime/debug/stack.go:24 +0x5e
kitty/tools/tui/loop.(*Loop).Run.func1()
	kitty/tools/tui/loop/api.go:244 +0x66
panic({0x562c44ea9880?, 0xc000420000?})
	runtime/panic.go:914 +0x21f
kitty/kittens/diff.render(0xc000418a20?, 0xc0004320f0, {0x2c, 0xf0, 0x2b, 0x8, 0x18}, 0x562c4454fca6?, {0x0, 0x0})
	kitty/kittens/diff/render.go:755 +0x232
kitty/kittens/diff.(*Handler).render_diff(0xc0002003c0)
	kitty/kittens/diff/ui.go:301 +0xad
kitty/kittens/diff.(*Handler).handle_async_result(0xc0002003c0, {{0x0, 0x0}, 0x1, 0x0, 0xc0004320f0, {0x0, 0x0}})
	kitty/kittens/diff/ui.go:256 +0x245
kitty/kittens/diff.(*Handler).on_wakeup(0xc0002003c0)
	kitty/kittens/diff/ui.go:169 +0xe5
kitty/tools/tui/loop.(*Loop).run(0xc0002001e0)
	kitty/tools/tui/loop/run.go:433 +0x142e
kitty/tools/tui/loop.(*Loop).Run(0x0?)
	kitty/tools/tui/loop/api.go:261 +0x65
kitty/kittens/diff.main(0xc000150000?, 0x562c44e0eb40?, {0xc000115a80, 0x2, 0xc000291528?})
	kitty/kittens/diff/main.go:161 +0x61b
kitty/kittens/diff.create_cmd.func1(0x562c44e3f300?, {0xc000115a80, 0x2, 0x8})
	kitty/kittens/diff/cli_generated.go:15 +0x8e
kitty/tools/cli.(*Command).ExecArgs(0x562c44ab992c?, {0xc000024080?, 0xc000150a00?, 0xc000186000?})
	kitty/tools/cli/command.go:549 +0x2b4
kitty/tools/cli.(*Command).Exec(0xc000186000?, {0x0?, 0x562c44ac2200?, 0xc00009e058?})
	kitty/tools/cli/command.go:567 +0x32
main.main()
	kitty/tools/cmd/main.go:33 +0x10d
Press any key to exit.

Screenshots
N/A

Environment details

kitty 0.29.2 created by Kovid Goyal
Linux cavejohnson 6.4.12-arch1-1 #1 SMP PREEMPT_DYNAMIC Thu, 24 Aug 2023 00:38:14 +0000 x86_64
Arch Linux 6.4.12-arch1-1 (/dev/tty)

Running under: X11
Frozen: False
Paths:
  kitty: /usr/bin/kitty
  base dir: /usr/lib/kitty
  extensions dir: /usr/lib/kitty/kitty
  system shell: /bin/fish
Loaded config files:
  /home/elliot/dotfiles/.config/kitty/kitty.conf
  /home/elliot/dotfiles/.config/kitty/condensed.conf

Config options different from defaults:
allow_remote_control       yes
copy_on_select             clipboard
cursor_blink_interval      0.0
cursor_shape               3
cursor_stop_blinking_after 4.0
draw_minimal_borders       False
font_family                IntelOne Mono
font_size                  10.0
hide_window_decorations    1
italic_font                Meslo LGM Nerd Font
modify_font:
    cell_height 5
mouse_hide_wait            1.0
shell_integration          frozenset({'no-cursor'})
single_window_margin_width FloatEdges(left=0.0, top=0.0, right=0.0, bottom=0.0)
visual_bell_duration       0.5
window_border_width        (0, 'pt')
Colors:
	background                 #202020   
	color0                     #151515   
	color1                     #ac4142   
	color10                    #7e8d50   
	color11                    #e5b566   
	color12                    #6c99ba   
	color13                    #9e4e85   
	color14                    #7dd5cf   
	color15                    #f5f5f5   
	color2                     #7e8d50   
	color3                     #e5b566   
	color4                     #6c99ba   
	color5                     #9e4e85   
	color6                     #7dd5cf   
	color7                     #d0d0d0   
	color8                     #505050   
	color9                     #ac4142   
	cursor                     #d0d0d0   
	foreground                 #d0d0d0   
	selection_background       #eecb8b   
	selection_foreground       #232323   

Important environment variables seen by the kitty process:
	PATH                                /home/elliot/.asdf/shims:/opt/asdf-vm/bin:/home/elliot/.local/share/gem/ruby/3.1.0/bin:/home/elliot/.local/share/gem/ruby/3.0.0/bin:/home/elliot/.local/share/gem/ruby/3.1.0:/home/elliot/.cargo/bin:/home/elliot/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl
	LANG                                en_US.UTF-8
	EDITOR                              nvim
	SHELL                               /bin/fish
	DISPLAY                             :0
	USER                                elliot
	XDG_SESSION_CLASS                   user
	XDG_RUNTIME_DIR                     /run/user/1000
	XDG_SESSION_ID                      1
	XDG_VTNR                            1
	XDG_SESSION_TYPE                    tty
	XDG_SEAT                            seat0

Additional context
The problem persists in a kitty instance initialized with kitty --config NONE

[kovidgoyal]

Well actually that commit wont be enough, will need more work

[kovidgoyal]

Basically the diff consists only of mode changes which the diff kitten doesnt support. The empty changeset is causing the panic, I can prevent that easily enough but ideally it should show the mode change

[elliotwesoff]

Thank you so much!