Clean up work-tree handling

The old version of work-tree support was an unholy mess, barely readable,
and not to the point.

For example, why do you have to provide a worktree, when it is not used?
As in "git status".  Now it works.

Another riddle was: if you can have work trees inside the git dir, why
are some programs complaining that they need a work tree?

IOW it is allowed to call

	$ git --git-dir=../ --work-tree=. bla

when you really want to.  In this case, you are both in the git directory
and in the working tree.  So, programs have to actually test for the right
thing, namely if they are inside a working tree, and not if they are
inside a git directory.

Also, GIT_DIR=../.git should behave the same as if no GIT_DIR was
specified, unless there is a repository in the current working directory.
It does now.

The logic to determine if a repository is bare, or has a work tree
(tertium non datur), is this:

--work-tree=bla overrides GIT_WORK_TREE, which overrides core.bare = true,
which overrides core.worktree, which overrides GIT_DIR/.. when GIT_DIR
ends in /.git, which overrides the directory in which .git/ was found.

In related news, a long standing bug was fixed: when in .git/bla/x.git/,
which is a bare repository, git formerly assumed ../.. to be the
appropriate git dir.  This problem was reported by Shawn Pearce to have
caused much pain, where a colleague mistakenly ran "git init" in "/" a
long time ago, and bare repositories just would not work.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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