Merge branch 'mh/rebase-fixup' (early part)

* 'mh/rebase-fixup' (early part): rebase-i: Ignore comments and blank lines in peek_next_command lib-rebase: Allow comments and blank lines to be added to the rebase script lib-rebase: Provide clearer debugging info about what the editor did Add a command "fixup" to rebase --interactive t3404: Use test_commit to set up test repository