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 () {

t/t3404-rebase-interactive.sh

@@ -265,6 +265,30 @@ test_expect_success 'squash and fixup generate correct log messages' '
 	git branch -D squash-fixup
 '
 
+test_expect_success 'squash ignores comments' '
+	git checkout -b skip-comments E &&
+	base=$(git rev-parse HEAD~4) &&
+	FAKE_COMMIT_AMEND="ONCE" FAKE_LINES="# 1 # squash 2 # squash 3 # squash 4 #" \
+		EXPECT_HEADER_COUNT=4 \
+		git rebase -i $base &&
+	test $base = $(git rev-parse HEAD^) &&
+	test 1 = $(git show | grep ONCE | wc -l) &&
+	git checkout to-be-rebased &&
+	git branch -D skip-comments
+'
+
+test_expect_success 'squash ignores blank lines' '
+	git checkout -b skip-blank-lines E &&
+	base=$(git rev-parse HEAD~4) &&
+	FAKE_COMMIT_AMEND="ONCE" FAKE_LINES="> 1 > squash 2 > squash 3 > squash 4 >" \
+		EXPECT_HEADER_COUNT=4 \
+		git rebase -i $base &&
+	test $base = $(git rev-parse HEAD^) &&
+	test 1 = $(git show | grep ONCE | wc -l) &&
+	git checkout to-be-rebased &&
+	git branch -D skip-blank-lines
+'
+
 test_expect_success 'squash works as expected' '
 	for n in one two three four
 	do