feat(gitsigns): new keymap for Gitsigns blame (#3758)

Also add filetype `gitsigns.blame` to autocmd `lazyvim_close_with_q` and
workaround for `+hunks` group name to show prefix correctly. If you
don't like the workaround for `+hunks` group name feel free to remove
it.

## What is this PR for?
Make use of the new `gitsigns` feature `Gitsigns blame`, which shows in
a vertical buffer split the `blame` history of the whole buffer.
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  why we should accept this pull request. -->

## Does this PR fix an existing issue?
No.
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  If this PR fixes any issues, please link to the issue here.
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## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

lua/lazyvim/config/autocmds.lua

@@ -67,6 +67,7 @@ vim.api.nvim_create_autocmd("FileType", {
     "neotest-summary",
     "neotest-output-panel",
     "dbout",
+    "gitsigns.blame",
   },
   callback = function(event)
     vim.bo[event.buf].buflisted = false