[BUG?] worktree setup broken in subdirs with git alias

Hi there,

sorry I can't dig deeper now, but the worktree code from next seems to
get confused now as soon as you cd to a subdir of a worktree (other than
the main worktree) and use an alias:

git help ss
`git ss' is aliased to `status -s -b'
[mjg@skimbleshanks Biomath-WS15 (exam $)]✓ git status -s -b
## exam
[mjg@skimbleshanks Biomath-WS15 (exam $)]✓ git ss
fatal: internal error: work tree has already been set
Current worktree: /home/mjg/Teaching/LUK
New worktree: /home/mjg/Teaching/LUK/Biomath-WS15

This is inside the subdir "Biomath-WS15" of the worktree residing in
".../LUK".

It wasn't like that last year ;)

Something about setting GIT_DIR and the like in the environment must
have changed (for aliases), badly interacting with the worktree code.

Michael