git-gui: correct assignment of work-tree

git-gui: correct assignment of work-tree

git-gui currently uses its own logic to determine the work-tree setting
but 'git rev-parse --toplevel' directly returns git's work-tree value
by calling get_git_work_tree() and is therefore always correct.

This fixes an inability to handle some repository configurations. In
particular where .git is a file containing a path to the real directory
(a cross-platform symbolic link).
To continue to support older versions than 1.7.0, setting the work-tree
by normalizing the --show-cdup value is more reliable as git-dir might be
outside the work-tree entirely.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>