add 'merge.stat' config variable

This variable has the same effect, as 'merge.diffstat'.
Also mention it in the documentation.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-config.txt

@@ -1,3 +1,7 @@
+merge.stat::
+	Whether to print the diffstat berween ORIG_HEAD and merge result
+	at the end of the merge.  True by default.
+
 merge.summary::
 	Whether to include summaries of merged commits in newly created
 	merge commit messages. False by default.

Documentation/merge-options.txt

@@ -1,6 +1,6 @@
 --stat::
 	Show a diffstat at the end of the merge. The diffstat is also
-	controlled by the configuration option merge.diffstat.
+	controlled by the configuration option merge.stat.
 
 -n, \--no-stat::
 	Do not show diffstat at the end of the merge.

git-merge.sh

@@ -212,6 +212,7 @@ while test $args_left -lt $#; do shift; done
 
 if test -z "$show_diffstat"; then
     test "$(git config --bool merge.diffstat)" = false && show_diffstat=false
+    test "$(git config --bool merge.stat)" = false && show_diffstat=false
     test -z "$show_diffstat" && show_diffstat=t
 fi