git-gui: Completely remove support for creating octopus merges

git-gui: Completely remove support for creating octopus merges

I'm working on refactoring the UI of the merge dialog, because as it
currently stands the dialog is absolutely horrible, especially when
you have 200+ branches available from a single remote system.

In that refactoring I plan on using the choose_rev widget to allow
the user to select exactly which branch/commit they want to merge.
However since that only selects a single commit I'm first removing
the code that supports octopus merges.

A brief consultation on #git tonight seemed to indicate that the
octopus merge strategy is not as useful as originally thought when
it was invented, and that most people don't commonly use them.  So
making users fall back to the command line to create an octopus is
actually maybe a good idea here, as they might think twice before
they use it.

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