am, rebase--merge: do not overlook --skip'ed commits with post-rewrite

The post-rewrite hook is supposed to be invoked for each rewritten
commit.  The fact that a commit was selected and processed by the rebase
operation (even though when we hit an error a user said it had no more
useful changes), suggests we should write an entry for it.  In
particular, let's treat it as an empty commit trivially squashed into
its parent.

This brings the rebase--am and rebase--merge backends in sync with the
behavior of the interactive rebase backend.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -2000,6 +2000,15 @@ static void am_skip(struct am_state *state)
 	if (clean_index(&head, &head))
 		die(_("failed to clean index"));
 
+	if (state->rebasing) {
+		FILE *fp = xfopen(am_path(state, "rewritten"), "a");
+
+		assert(!is_null_oid(&state->orig_commit));
+		fprintf(fp, "%s ", oid_to_hex(&state->orig_commit));
+		fprintf(fp, "%s\n", oid_to_hex(&head));
+		fclose(fp);
+	}
+
 	am_next(state);
 	am_load(state);
 	am_run(state, 0);

git-rebase--merge.sh

@@ -121,6 +121,8 @@ continue)
 skip)
 	read_state
 	git rerere clear
+	cmt="$(cat "$state_dir/cmt.$msgnum")"
+	echo "$cmt $(git rev-parse HEAD^0)" >> "$state_dir/rewritten"
 	msgnum=$(($msgnum + 1))
 	while test "$msgnum" -le "$end"
 	do

t/t5407-post-rewrite-hook.sh

@@ -78,6 +78,7 @@ test_expect_success 'git rebase --skip' '
 	git rebase --continue &&
 	echo rebase >expected.args &&
 	cat >expected.data <<-EOF &&
+	$(git rev-parse C) $(git rev-parse HEAD^)
 	$(git rev-parse D) $(git rev-parse HEAD)
 	EOF
 	verify_hook_input
@@ -91,6 +92,7 @@ test_expect_success 'git rebase --skip the last one' '
 	echo rebase >expected.args &&
 	cat >expected.data <<-EOF &&
 	$(git rev-parse E) $(git rev-parse HEAD)
+	$(git rev-parse F) $(git rev-parse HEAD)
 	EOF
 	verify_hook_input
 '
@@ -120,6 +122,7 @@ test_expect_success 'git rebase -m --skip' '
 	git rebase --continue &&
 	echo rebase >expected.args &&
 	cat >expected.data <<-EOF &&
+	$(git rev-parse C) $(git rev-parse HEAD^)
 	$(git rev-parse D) $(git rev-parse HEAD)
 	EOF
 	verify_hook_input