feat(plugins): nvim.difftool can compare directories #35448

Problem:
Built-in diff mode (nvim -d) does not support directory diffing
as required by git difftool -d. This makes it difficult to compare
entire directories, detect renames, and navigate changes efficiently.

Solution:
Add a DiffTool plugin and command that enables side-by-side diffing of
files and directories in Neovim. The plugin supports rename detection,
highlights changes in the quickfix list, and provides a user command for
easy invocation. This allows proper integration with git difftool -d for
directory comparison.

Example git config:

```ini
[diff]
    tool = nvim_difftool

[difftool "nvim_difftool"]
    cmd = nvim -c "packadd nvim.difftool" -c "DiffTool $LOCAL $REMOTE"
```

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>
Co-authored-by: Phạm Bình An <111893501+brianhuster@users.noreply.github.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/functional/plugin/difftool_spec.lua

@@ -0,0 +1,64 @@
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local clear = n.clear
+local command = n.command
+local eq = t.eq
+local fn = n.fn
+
+local pathsep = n.get_pathsep()
+local testdir_left = 'Xtest-difftool-left'
+local testdir_right = 'Xtest-difftool-right'
+
+setup(function()
+  n.mkdir_p(testdir_left)
+  n.mkdir_p(testdir_right)
+  t.write_file(testdir_left .. pathsep .. 'file1.txt', 'hello')
+  t.write_file(testdir_left .. pathsep .. 'file2.txt', 'foo')
+  t.write_file(testdir_right .. pathsep .. 'file1.txt', 'hello world') -- modified
+  t.write_file(testdir_right .. pathsep .. 'file3.txt', 'bar') -- added
+end)
+
+teardown(function()
+  n.rmdir(testdir_left)
+  n.rmdir(testdir_right)
+end)
+
+describe('nvim.difftool', function()
+  before_each(function()
+    clear()
+    command('packadd nvim.difftool')
+  end)
+
+  it('shows added, modified, and deleted files in quickfix', function()
+    command(('DiffTool %s %s'):format(testdir_left, testdir_right))
+    local qflist = fn.getqflist()
+    local entries = {}
+    for _, item in ipairs(qflist) do
+      table.insert(entries, { text = item.text, rel = item.user_data and item.user_data.rel })
+    end
+
+    -- Should show:
+    -- file1.txt as modified (M)
+    -- file2.txt as deleted (D)
+    -- file3.txt as added (A)
+    eq({
+      { text = 'M', rel = 'file1.txt' },
+      { text = 'D', rel = 'file2.txt' },
+      { text = 'A', rel = 'file3.txt' },
+    }, entries)
+  end)
+
+  it('has autocmds when diff window is opened', function()
+    command(('DiffTool %s %s'):format(testdir_left, testdir_right))
+    local autocmds = fn.nvim_get_autocmds({ group = 'nvim.difftool.events' })
+    assert(#autocmds > 0)
+  end)
+
+  it('cleans up autocmds when diff window is closed', function()
+    command(('DiffTool %s %s'):format(testdir_left, testdir_right))
+    command('q')
+    local ok = pcall(fn.nvim_get_autocmds, { group = 'nvim.difftool.events' })
+    eq(false, ok)
+  end)
+end)