builtin/init: simplify logic to configure worktree

builtin/init: simplify logic to configure worktree

In the preceding commit we have stopped modifying the global
`git_work_tree_cfg` variable. With this change there's now some code
paths where we end up setting the local `git_work_tree_cfg` variable,
but without actually using the value for anything.

Refactor the code a bit so that we only set the worktree configuration
in case it's actually needed. Furthermore, reflow it a bit to make the
code easier to follow.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>