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>

git-gui.sh

@@ -2693,10 +2693,6 @@ proc start_local_merge_action {w} {
 	global HEAD ui_status_value current_branch
 
 	set cmd [list git merge]
-	if {![is_config_true merge.summary]} {
-		lappend cmd --no-summary
-	}
-
 	set names {}
 	set revcnt 0
 	foreach i [$w.source.l curselection] {
@@ -3772,7 +3768,7 @@ proc do_options {} {
 
 	set optid 0
 	foreach option {
-		{b merge.summary {Show Merge Summary}}
+		{b merge.summary {Summarize Merge Commits}}
 		{i-1..5 merge.verbosity {Merge Verbosity}}
 
 		{b gui.trustmtime  {Trust File Modification Timestamps}}
@@ -4153,7 +4149,7 @@ proc apply_config {} {
 	}
 }
 
-set default_config(merge.summary) true
+set default_config(merge.summary) false
 set default_config(merge.verbosity) 2
 set default_config(gui.trustmtime) false
 set default_config(gui.diffcontext) 5