fix(difftool): fully resolve symlinks when comparing paths #36147

Fixes issue on mac where it was constantly reloading buffers as paths
were not being normalized and resolved correctly (in relation to buffer
name).

Quickfix entry:
/var/folders/pt/2s7dzyw12v36tsslrghfgpkr0000gn/T/git-difftool.m95lj8/right/app.vue

Buffer name:
/private/var/folders/pt/2s7dzyw12v36tsslrghfgpkr0000gn/T/git-difftool.m95lj8/right/app.vue

/var was synlinked to /private/var and this was not being properly
handled.

Also added lazy redraw to avoid too many redraws when this happens in
future and added test for symlink handling.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

test/functional/plugin/difftool_spec.lua

@@ -61,6 +61,31 @@ describe('nvim.difftool', function()
     )
   end)
 
+  it('handles symlinks', function()
+    -- Create a symlink in right dir pointing to file2.txt in left dir
+    local symlink_path = vim.fs.joinpath(testdir_right, 'file2.txt')
+    local target_path = vim.fs.joinpath('..', testdir_left, 'file2.txt')
+    assert(vim.uv.fs_symlink(target_path, symlink_path) == true)
+    finally(function()
+      os.remove(symlink_path)
+    end)
+    assert(fn.getftype(symlink_path) == 'link')
+
+    -- Run difftool
+    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
+
+    -- file2.txt should not be reported as added or deleted anymore
+    eq({
+      { text = 'M', rel = 'file1.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' })