Use simpler relative_path when set_git_dir

Use simpler relative_path when set_git_dir

Using a relative_path as git_dir first appears in v1.5.6-1-g044bbbc.
It will make git_dir shorter only if git_dir is inside work_tree,
and this will increase performance. But my last refactor effort on
relative_path function (commit v1.8.3-rc2-12-ge02ca72) changed that.
Always use relative_path as git_dir may bring troubles like
$gmane/234434.

Because new relative_path is a combination of original relative_path
from path.c and original path_relative from quote.c, so in order to
restore the origin implementation, save the original relative_path
as remove_leading_path, and call it in setup.c.

Suggested-by: Karsten Blees <karsten.blees@gmail.com>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>