rebase -i: interrupt rebase when "commit --amend" failed during "reword"

"commit --amend" could fail in cases like the user empties the commit
message, or pre-commit failed.  When it fails, rebase should be
interrupted and alert the user, rather than ignoring the error and
continue on rebasing.  This also gives users a way to gracefully
interrupt a "reword" if they decided they actually want to do an "edit",
or even "rebase --abort".

Signed-off-by: Andrew Wong <andrew.kw.w@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -143,6 +143,21 @@ die_with_patch () {
 	die "$2"
 }
 
+exit_with_patch () {
+	echo "$1" > "$state_dir"/stopped-sha
+	make_patch $1
+	git rev-parse --verify HEAD > "$amend"
+	warn "You can amend the commit now, with"
+	warn
+	warn "	git commit --amend"
+	warn
+	warn "Once you are satisfied with your changes, run"
+	warn
+	warn "	git rebase --continue"
+	warn
+	exit $2
+}
+
 die_abort () {
 	rm -rf "$state_dir"
 	die "$1"
@@ -395,7 +410,13 @@ do_next () {
 		mark_action_done
 		pick_one $sha1 ||
 			die_with_patch $sha1 "Could not apply $sha1... $rest"
-		git commit --amend --no-post-rewrite
+		git commit --amend --no-post-rewrite || {
+			warn "Could not amend commit after successfully picking $sha1... $rest"
+			warn "This is most likely due to an empty commit message, or the pre-commit hook"
+			warn "failed. If the pre-commit hook failed, you may need to resolve the issue before"
+			warn "you are able to reword the commit."
+			exit_with_patch $sha1 1
+		}
 		record_in_rewritten $sha1
 		;;
 	edit|e)
@@ -404,19 +425,8 @@ do_next () {
 		mark_action_done
 		pick_one $sha1 ||
 			die_with_patch $sha1 "Could not apply $sha1... $rest"
-		echo "$sha1" > "$state_dir"/stopped-sha
-		make_patch $sha1
-		git rev-parse --verify HEAD > "$amend"
 		warn "Stopped at $sha1... $rest"
-		warn "You can amend the commit now, with"
-		warn
-		warn "	git commit --amend"
-		warn
-		warn "Once you are satisfied with your changes, run"
-		warn
-		warn "	git rebase --continue"
-		warn
-		exit 0
+		exit_with_patch $sha1 0
 		;;
 	squash|s|fixup|f)
 		case "$command" in