rebase -i: fix post-rewrite hook with failed exec command

Usually, when 'git rebase' stops before completing the rebase, it is to
give the user an opportunity to edit a commit (e.g. with the 'edit'
command). In such cases, 'git rebase' leaves the sha1 of the commit being
rewritten in "$state_dir"/stopped-sha, and subsequent 'git rebase
--continue' will call the post-rewrite hook with this sha1 as <old-sha1>
argument to the post-rewrite hook.

The case of 'git rebase' stopping because of a failed 'exec' command is
different: it gives the opportunity to the user to examine or fix the
failure, but does not stop saying "here's a commit to edit, use
--continue when you're done". So, there's no reason to call the
post-rewrite hook for 'exec' commands. If the user did rewrite the
commit, it would be with 'git commit --amend' which already called the
post-rewrite hook.

Fix the behavior to leave no stopped-sha file in case of failed exec
command, and teach 'git rebase --continue' to skip record_in_rewritten if
no stopped-sha file is found.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -486,7 +486,7 @@ do_pick () {
 }
 
 do_next () {
-	rm -f "$msg" "$author_script" "$amend" || exit
+	rm -f "$msg" "$author_script" "$amend" "$state_dir"/stopped-sha || exit
 	read -r command sha1 rest < "$todo"
 	case "$command" in
 	"$comment_char"*|''|noop)
@@ -576,9 +576,6 @@ do_next () {
 		read -r command rest < "$todo"
 		mark_action_done
 		printf 'Executing: %s\n' "$rest"
-		# "exec" command doesn't take a sha1 in the todo-list.
-		# => can't just use $sha1 here.
-		git rev-parse --verify HEAD > "$state_dir"/stopped-sha
 		${SHELL:-@SHELL_PATH@} -c "$rest" # Actual execution
 		status=$?
 		# Run in subshell because require_clean_work_tree can die.
@@ -874,7 +871,10 @@ first and then run 'git rebase --continue' again."
 		fi
 	fi
 
-	record_in_rewritten "$(cat "$state_dir"/stopped-sha)"
+	if test -r "$state_dir"/stopped-sha
+	then
+		record_in_rewritten "$(cat "$state_dir"/stopped-sha)"
+	fi
 
 	require_clean_work_tree "rebase"
 	do_rest

t/t5407-post-rewrite-hook.sh

@@ -212,7 +212,7 @@ EOF
 	verify_hook_input
 '
 
-test_expect_failure 'git rebase -i (exec)' '
+test_expect_success 'git rebase -i (exec)' '
 	git reset --hard D &&
 	clear_hook_input &&
 	FAKE_LINES="edit 1 exec_false 2" git rebase -i B &&