git-gui: Reword meaning of merge.summary.

git-gui: Reword meaning of merge.summary.

OK, its official, I'm not reading documentation as well as I should be.
Core Git's merge.summary configuration option is used to control the
generation of the text appearing within the merge commit itself.  It
is not (and never has been) used to default the --no-summary command
line option, which disables the diffstat at the end of the merge.

I completely blame Git for naming two unrelated options almost the
exact same thing.  But its my own fault for allowing git-gui to
confuse the two.

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