meson: Copy files with git only in true git repository

meson: Copy files with git only in true git repository

When mkosi is run from git-worktree(1), the .git is not a repository
directory but a textfile pointing to the real git dir
(e.g. /home/user/systemd/.git/worktrees/systemd-worktree). This git dir
is not bind mounted into build environment and it fails with:

> fatal: not a git repository: /home/user/systemd/.git/worktrees/systemd-worktree
> test/meson.build:190:16: ERROR: Command `/usr/bin/env -u GIT_WORK_TREE /usr/bin/git --git-dir=/root/src/.git ls-files ':/test/dmidecode-dumps/*.bin'` failed with status 128.

There is already a fallback to use shell globbing instead of ls-files,
use it with git worktrees as well.