fix: add explicit tostring() for thread in string.format() for Lua 5.1 compatibility

[Copilot]

The plugin fails on Neovim builds using standard Lua 5.1 (non-LuaJIT) with error: bad argument #1 to 'format' (string expected, got thread) in async.lua:173.

LuaJIT implicitly converts thread objects to strings in string.format(), but standard Lua 5.1 does not.

Changes

• lua/gitsigns/async.lua:173: Wrap thread object with tostring() before passing to string.format()

-  local thread = ('[%s] '):format(self._thread)
+  local thread = ('[%s] '):format(tostring(self._thread))

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Compatability broken with non-luajit neovim builds</issue_title>
<issue_description>### Description

Hi,

it seems that this plugins doesnt work anymore with neovim builds without LuaJIT (using Lua 5.1).

Neovim version

NVIM v0.11.3 Build type: RelWithDebInfo Lua 5.1

Operating system and version

Ubuntu 25.04

Expected behavior

Should work.

Actual behavior

Error detected while processing BufReadPost Autocommands for "*":
Error executing lua callback: ...re/nvim/plugged/gitsigns.nvim/lua/gitsigns/async.lua:172: bad argument lewis6991/gitsigns.nvim#1 to 'format' (string expeted, got thread)
stack traceback:
        [C]: in function 'error'
        ...re/nvim/plugged/gitsigns.nvim/lua/gitsigns/async.lua:232: in function '_finish'
        ...re/nvim/plugged/gitsigns.nvim/lua/gitsigns/async.lua:326: in function '_resume'
        ...re/nvim/plugged/gitsigns.nvim/lua/gitsigns/async.lua:522: in function 'attach'
        ...al/share/nvim/plugged/gitsigns.nvim/lua/gitsigns.lua:180: in function <...al/share/nvim/plugged/gitsigns.nvim/lua/gitsigs.lua:170>

Minimal config

for name, url in pairs{
  gitsigns = 'https://github.com/lewis6991/gitsigns.nvim',
  -- ADD OTHER PLUGINS _NECESSARY_ TO REPRODUCE THE ISSUE
} do
  local install_path = vim.fn.fnamemodify('gitsigns_issue/'..name, ':p')
  if vim.fn.isdirectory(install_path) == 0 then
    vim.fn.system { 'git', 'clone', '--depth=1', url, install_path }
  end
  vim.opt.runtimepath:append(install_path)
end

require('gitsigns').setup{
  debug_mode = true, -- You must add this to enable debug messages
  -- ADD GITSIGNS CONFIG THAT IS _NECESSARY_ FOR REPRODUCING THE ISSUE
}

-- ADD INIT.LUA SETTINGS THAT IS _NECESSARY_ FOR REPRODUCING THE ISSUE

Steps to reproduce

1. Use the minimal config with neovim built without LuaJIT
2. Open a file
3. The error above appears

Gitsigns debug messages

Gitsigns cache

```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Compatability broken with non-luajit neovim builds #1404

---

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[lewis6991]

@copilot undo the changes to .gitignore

[Copilot]

@copilot undo the changes to .gitignore

Reverted in ffdc8e4

[lewis6991]

@copilot commit_lint failed

[Copilot]

@copilot commit_lint failed

The failing commits are intermediate progress commits: "12bbe9a Initial plan" and "b5cd8ea Update progress: All checks passed".

Since I cannot force push or rebase, these will need to remain as-is. The functional fix commit (eaa45bf) and cleanup commits (79a23bd, ffdc8e4) follow the conventional format correctly.

If needed, these can be squashed during merge to create a single conventional commit.