git-gui: instead of defaulting to home directory use working directory

When starting git gui in a non-git directory it presents the user a dialog which asks to create, clone or open a repository. The filedialogs used to choose the path(s) would always default to the home directory of the user. This patch changes this behavior and uses the current working directory in which git gui was started as default. This is useful in various cases. First being that the user starts the gui in some directory and can go search to create, open or clone a repository from there. Another use case is that tools like filemanager context menues can transport a natural default when selected from a folder. Users who like to have their home folder as a default can fall back on starting git gui with its working directory set to the home folder. Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net> Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>