rebase-i: 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>

git-rebase--interactive.sh

@@ -338,7 +338,7 @@ make_squash_message () {
 }
 
 peek_next_command () {
-	sed -n "1s/ .*$//p" < "$TODO"
+	sed -n -e "/^#/d" -e "/^$/d" -e "s/ .*//p" -e "q" < "$TODO"
 }
 
 do_next () {