fix: avoid crash when closing the tab while still updating

[raw1z]

When diffview's tab is closed while the commit list is still updating (update can take a few seconds for big repositories), we get an error:

Error executing vim.schedule lua callback: ...diffview/scene/views/file_history/file_history_panel.lua:214: attempt to index field 'option_panel' (a nil value)
stack traceback:
        ...diffview/scene/views/file_history/file_history_panel.lua:214: in function <...diffview/scene/views/file_history/file_history_panel.lua:214>

[sindrets]

LGTM, thanks!

[sindrets]

(update can take a few seconds for big repositories)

@raw1z Multiple seconds? Is that with the default commit limit of 256? What OS is this? For reference, my 6+ year old laptop uses ~500ms to load history for repositories with complex git histories, and ~200ms otherwise.

[raw1z]

I'm on macos. I have an intel mac and a m1 mac. I experience the multiple seconds delay only on the m1 mac for the same repo (https://gitlab.com/raw1z/vim)