Initialize the new gem's git repo without a subshell

`bundle gem` ran `git init` through a backtick subshell with POSIX shell
quoting. On Windows the backtick invokes cmd.exe, which ignores the POSIX
escaping, so a target path containing spaces was split and the repo was
created in the wrong place. Spawn git directly with an argument array,
matching the `git add` call below.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hsbt

lib/bundler/cli/gem.rb

@@ -247,8 +247,7 @@ def run
 
       if use_git
         Bundler.ui.info "\nInitializing git repo in #{target}"
-        require "shellwords"
-        `git init #{target.to_s.shellescape}`
+        IO.popen(["git", "init", target.to_s], &:read)
 
         config[:git_default_branch] = File.read("#{target}/.git/HEAD").split("/").last.chomp
       end

spec/bundler/commands/newgem_spec.rb

@@ -88,8 +88,6 @@ def installed_go?
       end
 
       it "properly initializes git repo" do
-        skip "path with spaces needs special handling on Windows" if Gem.win_platform?
-
         bundle "gem #{gem_name}", dir: bundled_app("path with spaces")
         expect(bundled_app("path with spaces/#{gem_name}/.git")).to exist
       end