rebase--interactive: Ignore comments and blank lines in peek_next_command

Previously, blank lines and/or comments within a series of squash/fixup commands would confuse "git rebase -i" into thinking that the series was finished. It would therefore require the user to edit the commit message for the squash/fixup commits seen so far. Then, after continuing, it would ask the user to edit the commit message again. Ignore comments and blank lines within a group of squash/fixup commands, allowing them to be processed in one go. Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>