fix(select_hunk): compatible with <cmd> mapping

[xzbdmw]

Previous type vih will only select hunk lines which are below current line with mapping like
map({ "o", "x" }, "ih", "<cmd>Gitsigns select_hunk<CR>").
Also update the readme to use the faster <cmd> mapping.

[lewis6991]

Can you explain an example this fixes?

I can't remember exactly why, but I think I specifically avoided using <cmd> for this mapping.

[xzbdmw]

iShot_2025-02-04_23.43.44.mp4

In this video I'm typing vih with <cmd> mapping.

I can't remember exactly why, but I think I specifically avoided using for this mapping.

This pr also works for lua functions such as

                map({ "o", "x" }, "ih", function()
                    gs.select_hunk()
                end)

[lewis6991]

vih works fine for me. I'm not sure what this PR fixes.

[xzbdmw]

It only works if the mapping uses : to enter command mode, but not when the mapping is function callback form or cmd form. Here is a video show why the original one does not work when start at visual mode, I'm simulating vim.cmd('normal! ' .. hunk.added.start .. 'GV' .. hunk.vend .. 'G') here:

iShot_2025-02-06_01.18.24.mp4

[lewis6991]

So this change doesn't actually fix anything it just makes <cmd> work?