Merge branch 'js/rebase-am-options'

Merge branch 'js/rebase-am-options'

The way "git rebase" parses and forwards the command line options
meant for underlying "git am" has been revamped, which fixed for
options with parameters that were not passed correctly.

* js/rebase-am-options:
  rebase: validate -C<n> and --whitespace=<mode> parameters early
  rebase: really just passthru the `git am` options