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