builtin/init: stop modifying global `git_work_tree_cfg` variable

builtin/init: stop modifying global `git_work_tree_cfg` variable

When executing git-init(1) we need to figure out the final location of
the worktree. This location can be configured in a couple of ways: via
an environment variable, via the preexisting "core.worktree" config in
case we're reinitializing, or implicitly when reinitializing a non-bare
repository.

When checking for the worktree location in "builtin/init-db.c" we
populate any potentially-discovered value both by setting the global
`git_work_tree_cfg` variable and via `set_git_work_tree()`, which
ultimately ends up modifying `struct repository::worktree`.

Modifying `git_work_tree_cfg` is unnecessary though: we configure the
worktree in `create_default_files()`, and that function derives the
worktree location via `repo_get_work_tree()`. Consequently, propagating
the worktree via `set_git_work_tree()` is sufficient.

Stop munging `git_work_tree_cfg` and make it file-local to "setup.c" and
function-local to `cmd_init_db()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>