This is crucial to improve performance on Windows, as the speed is now
mostly dominated by the SHA-1 transformation (because it spawns a new
rev-parse process for *every* line, and spawning processes is pretty
slow from Git for Windows' MSYS2 Bash).

Signed-off-by: Johannes Schindelin <redacted>
---
 builtin/rebase--helper.c   | 10 +++++++-
 git-rebase--interactive.sh | 27 ++--------------------
 sequencer.c                | 57 ++++++++++++++++++++++++++++++++++++++++++++++
 sequencer.h                |  2 ++
 4 files changed, 70 insertions(+), 26 deletions(-)

diff --git a/builtin/rebase--helper.c b/builtin/rebase--helper.c
index 821058d452d..9444c8d6c60 100644
--- a/builtin/rebase--helper.c
+++ b/builtin/rebase--helper.c

@@ -13,7 +13,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 	struct replay_opts opts = REPLAY_OPTS_INIT;
 	int keep_empty = 0;
 	enum {
-		CONTINUE = 1, ABORT, MAKE_SCRIPT
+		CONTINUE = 1, ABORT, MAKE_SCRIPT, SHORTEN_SHA1S, EXPAND_SHA1S
 	} command = 0;
 	struct option options[] = {
 		OPT_BOOL(0, "ff", &opts.allow_ff, N_("allow fast-forward")),

@@ -24,6 +24,10 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 				ABORT),
 		OPT_CMDMODE(0, "make-script", &command,
 			N_("make rebase script"), MAKE_SCRIPT),
+		OPT_CMDMODE(0, "shorten-sha1s", &command,
+			N_("shorten SHA-1s in the todo list"), SHORTEN_SHA1S),
+		OPT_CMDMODE(0, "expand-sha1s", &command,
+			N_("expand SHA-1s in the todo list"), EXPAND_SHA1S),

 		OPT_END()
 	};
 

@@ -42,5 +46,9 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 		return !!sequencer_remove_state(&opts);
 	if (command == MAKE_SCRIPT && argc > 1)
 		return !!sequencer_make_script(keep_empty, stdout, argc, argv);
+	if (command == SHORTEN_SHA1S && argc == 1)
+		return !!transform_todo_ids(1);
+	if (command == EXPAND_SHA1S && argc == 1)
+		return !!transform_todo_ids(0);
 	usage_with_options(builtin_rebase_helper_usage, options);
 }

diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 214af0372ba..82a1941c42c 100644
--- a/git-rebase--interactive.sh
+++ b/git-rebase--interactive.sh

@@ -750,35 +750,12 @@ skip_unnecessary_picks () {
 		die "$(gettext "Could not skip unnecessary pick commands")"
 }
 
-transform_todo_ids () {
-	while read -r command rest
-	do
-		case "$command" in
-		"$comment_char"* | exec)
-			# Be careful for oddball commands like 'exec'
-			# that do not have a SHA-1 at the beginning of $rest.
-			;;
-		*)
-			sha1=$(git rev-parse --verify --quiet "$@" ${rest%%[	 ]*}) &&
-			if test "a$rest" = "a${rest#*[	 ]}"
-			then
-				rest=$sha1
-			else
-				rest="$sha1 ${rest#*[	 ]}"
-			fi
-			;;
-		esac
-		printf '%s\n' "$command${rest:+ }$rest"
-	done <"$todo" >"$todo.new" &&
-	mv -f "$todo.new" "$todo"
-}
-
 expand_todo_ids() {
-	transform_todo_ids
+	git rebase--helper --expand-sha1s
 }
 
 collapse_todo_ids() {
-	transform_todo_ids --short
+	git rebase--helper --shorten-sha1s
 }
 
 # Rearrange the todo list that has both "pick sha1 msg" and

diff --git a/sequencer.c b/sequencer.c
index 88819a1a2a9..201d45b1677 100644
--- a/sequencer.c
+++ b/sequencer.c

@@ -2437,3 +2437,60 @@ int sequencer_make_script(int keep_empty, FILE *out,
 	strbuf_release(&buf);
 	return 0;
 }
+
+
+int transform_todo_ids(int shorten_sha1s)
+{
+	const char *todo_file = rebase_path_todo();
+	struct todo_list todo_list = TODO_LIST_INIT;
+	int fd, res, i;
+	FILE *out;
+
+	strbuf_reset(&todo_list.buf);
+	fd = open(todo_file, O_RDONLY);
+	if (fd < 0)
+		return error_errno(_("could not open '%s'"), todo_file);
+	if (strbuf_read(&todo_list.buf, fd, 0) < 0) {
+		close(fd);
+		return error(_("could not read '%s'."), todo_file);
+	}
+	close(fd);
+
+	res = parse_insn_buffer(todo_list.buf.buf, &todo_list);
+	if (res) {
+		todo_list_release(&todo_list);
+		return error(_("unusable instruction sheet: '%s'"), todo_file);

+			int len;
+
+			p += strspn(p, " \t"); /* left-trim command */
+			len = strcspn(p, " \t"); /* length of command */
+
+			fprintf(out, "%.*s %s %.*s\n",
+				len, p, sha1, item->arg_len, item->arg);
+		}
+	}
+	fclose(out);
+	todo_list_release(&todo_list);
+	return 0;
+}

diff --git a/sequencer.h b/sequencer.h
index 83f2943b7a9..47a81034e76 100644
--- a/sequencer.h
+++ b/sequencer.h

@@ -48,6 +48,8 @@ int sequencer_remove_state(struct replay_opts *opts);
 int sequencer_make_script(int keep_empty, FILE *out,
 		int argc, const char **argv);
 
+int transform_todo_ids(int shorten_sha1s);
+
 extern const char sign_off_header[];
 
 void append_signoff(struct strbuf *msgbuf, int ignore_footer, unsigned flag);

-- 
2.12.2.windows.2.800.gede8f145e06