git-gui: Support merge.summary, merge.verbosity.

git-gui: Support merge.summary, merge.verbosity.

Changed our private merge summary config option to be the same as the
merge.summary option supported by core Git.  This means setting the
"Show Merge Summary" flag in git-gui will have the same effect on
the command line.

In the same vein I've also added merge.verbosity to the gui options,
allowing the user to adjust the verbosity level of the recursive
merge strategy.  I happen to like level 1 and suggest that other users
use that, but level 2 is the core Git default right now so we'll use
the same default in git-gui.

Unfortunately it appears as though core Git has broken support for
the merge.summary option, even though its still in the documentation
For the time being we should pass along --no-summary to git-merge if
merge.summary is false.

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