git-gui: Added menu command to visualize all branches.

Sometimes its useful to start gitk with the --all option, to view all of
the known branches and tags within this repository.  Rather than making
the user startup gitk and then edit the view we can pass the option along
for them.

This also makes it slightly more explicit, that when gitk starts up by
default its showing the current branch and not everything.  Yes gitk
isn't showing that to the user, but the fact that the user had to make
a decision between seeing this current branch or all branches will
hopefully make them study gitk's display before jumping to a conclusion.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>