setup: do not forget working dir from subdir of gitdir

setup: do not forget working dir from subdir of gitdir

v1.6.1.3~4^2 (Fix gitdir detection when in subdir of gitdir,
2009-01-16) did not go far enough: when a git directory is
an ancestor of the original working directory, not only
should GIT_DIR be set to point to the .git directory, but
the original working directory should be restored before
carrying out the relevant command.

This way, the effect of running a git command from a subdir
of .git will be the same whether or not GIT_DIR is explicitly
set.

Noticed while investigating v1.6.0.3~1 (rehabilitate 'git
index-pack' inside the object store, 2008-10-20).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>