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

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>