git-gui: Factor out common fast-forward merge case

git-gui: Factor out common fast-forward merge case

In both the ff and reset merge_types supported by checkout_op the
result is the same if the merge base of our target commit and the
existing commit is the existing commit: its a fast-forward as the
existing commit is fully contained in the target commit.

This minor cleanup in logic will make it easier to implement a
new kind of merge_type that actually merges the two trees with a
real merge strategy, such as git-merge-recursive.

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