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