Avoid more expensive code path when no core.worktree is configured.
Don't change worktree when reading config if one is already set.
This could happen if GIT_CORE_WORKTREE is set, and the repo also has
core.worktree, and the config is reread. Now GIT_CORE_WORKTREE will
prevail.
The environment needs to override git-config. Changed when git config is
read, and avoid rereading it once it's been read.
chdir for both worktree settings.
