git-gui: Expose the merge.diffstat configuration option

Recently git-merge learned to avoid generating the diffstat after
a merge by reading the merge.diffstat configuration option.  By
default this option is assumed to be true, as that is the old
behavior.  However we can force it to false by setting it as a
standard boolean option.

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

git-gui.sh

@@ -1262,6 +1262,7 @@ proc apply_config {} {
 	}
 }
 
+set default_config(merge.diffstat) true
 set default_config(merge.summary) false
 set default_config(merge.verbosity) 2
 set default_config(user.name) {}

lib/option.tcl

@@ -184,6 +184,7 @@ proc do_options {} {
 
 		{b merge.summary {Summarize Merge Commits}}
 		{i-1..5 merge.verbosity {Merge Verbosity}}
+		{b merge.diffstat {Show Diffstat After Merge}}
 
 		{b gui.trustmtime  {Trust File Modification Timestamps}}
 		{b gui.pruneduringfetch {Prune Tracking Branches During Fetch}}