Merge branch 'js/maint-am-skip-performance-regression'

Recent versions of scripted "git am" has a performance regression in
"git am --skip" codepath, which no longer exists in the built-in
version on the 'master' front.  Fix the regression in the last
scripted version that appear in 2.5.x maintenance track and older.

* js/maint-am-skip-performance-regression:
  am --skip/--abort: merge HEAD/ORIG_HEAD tree into index

Author: gitster

contrib/examples/git-am.sh

@@ -512,7 +512,7 @@ then
 		git read-tree --reset -u $head_tree $head_tree &&
 		index_tree=$(git write-tree) &&
 		git read-tree -m -u $index_tree $head_tree
-		git read-tree $head_tree
+		git read-tree -m $head_tree
 		;;
 	,t)
 		if test -f "$dotest/rebasing"