git-gui: Save the merge base during checkout_op processing

git-gui: Save the merge base during checkout_op processing

I've decided to teach checkout_op how to perform more than just a
fast-forward and reset type of merge.  This way we can also do a full
recursive merge even when we are recreating an existing branch from
a remote.  To help with that process I'm saving the merge-base we
computed during the ff/reset/fail decision process, in case we need
it later on when we actually start a true merge operation.

Signed-off-by: Shawn O. Pearce <redacted>