git-prompt.sh: show the upstream abbrev name

When working with multiple remotes, it is common to switch the upstream
from a remote to another. Doing so, the prompt may not be the expected
one. Providing an option to display tracking information sounds useful.

Add a "name" option to GIT_PS1_SHOWUPSTREAM which will show the upstream
abbrev name. This option is ignored if "verbose" is false.

Signed-off-by: Julien Carsique <julien.carsique@gmail.com>