GIT_INDEX_FILE relative paths are relative to the work tree

["Jack O'Connor" <oconnor663@gmail.com>]

When I use a relative path in the GIT_INDEX_FILE environment variable,
git interprets that path relative to the the work tree. This can be
confusing if my cwd is some subdirectory of my project; in that case
an index file is created in the project root rather than in my cwd. It
can also be confusing if I'm using --git-dir and --work-tree, in which
case the path is interpreted relative to the latter.

Is this behavior intentional? If so, I think it would be worth
mentioning in the documentation for GIT_INDEX_FILE. But I can't think
of a case where I would ever want an index file to end up inside my
work tree.

This also comes up in http://stackoverflow.com/a/7645492/823869. Using
absolute paths is a workaround.