t7006: demonstrate a problem with aliases in subdirectories

t7006: demonstrate a problem with aliases in subdirectories

When expanding aliases, the git_dir is set during the alias expansion
(by virtue of running setup_git_directory_gently()).

This git_dir may be relative to the current working directory, and
indeed often is simply ".git/".

When the alias expands to a shell command, we restore the original
working directory, though, yet we do not reset git_dir.

As a consequence, subsequent read_early_config() runs will mistake the
git_dir to be populated properly and not find the correct config.

Demonstrate this problem by adding a test case.

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