git-gui: Use PWD if it exists on Mac OS X

git-gui: Use PWD if it exists on Mac OS X

The current working directory is set to / when git-gui is invoked
using the Git Gui.app bundle on Mac OS X. This means that if it is
launched from a directory which contains a repository then git-gui
won't automatically find it unless the repository happens to be
located in /.

The PWD environment variable is however preserved if the bundle is
invoked using open(1). If git-gui would check for PWD then a user
could for example type open -a 'Git Gui' on a command line in order to
launch the program and it would automatically find the repository.

Teach git-gui to use the PWD environment variable on Mac OS X.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>