Merge branch 'js/rebase-am-options-fix'

Recently, built-in "rebase" tightened the error checking for a few
options that are passed to underlying "am", but we forgot to make
the matching change to the scripted version, which has been
corrected.

* js/rebase-am-options-fix:
  legacy-rebase: backport -C<n> and --whitespace=<option> checks