fix: VCS checkout_vcs fails on CI (git show path resolution)

git show HEAD~0:./relative_path fails when CWD is not the git root.
Use git rev-parse --show-toplevel to get the actual repo root, then
compute the relative path from there. Removes the ./ prefix that
caused git to resolve paths relative to CWD instead of repo root.

Also ensure tmp/ directory exists before creating Tempfile in test.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pftg

lib/capybara/screenshot/diff/vcs.rb

@@ -9,9 +9,10 @@ module Vcs
         SILENCE_ERRORS = Os::ON_WINDOWS ? "2>nul" : "2>/dev/null"
 
         def self.checkout_vcs(root, screenshot_path, checkout_path)
-          vcs_file_path = screenshot_path.relative_path_from(root)
+          git_root = Pathname.new(`git rev-parse --show-toplevel 2>/dev/null`.chomp)
+          vcs_file_path = Pathname.new(screenshot_path).expand_path.relative_path_from(git_root)
           redirect_target = "#{checkout_path} #{SILENCE_ERRORS}"
-          show_command = "git show HEAD~0:./#{vcs_file_path}"
+          show_command = "git show HEAD:#{vcs_file_path}"
 
           Dir.chdir(root) do
             if Screenshot.use_lfs

test/unit/vcs_test.rb

@@ -9,6 +9,7 @@ class VcsTest < ActiveSupport::TestCase
         include Vcs
 
         setup do
+          FileUtils.mkdir_p(Screenshot.root)
           @base_screenshot = Tempfile.new(%w[vcs_base_screenshot. .attempt.0.png], Screenshot.root)
         end