rebase -i: fix short SHA-1 collision

The 'todo' sheet for interactive rebase shows abbreviated SHA-1's and
then performs its operations upon those shortened values. This can lead
to an abort if the SHA-1 of a reworded or edited commit is no longer
unique within the abbreviated SHA-1 space and a subsequent SHA-1 in the
todo list has the same abbreviated value.

For example:

  edit f00dfad first
  pick badbeef second

If, after editing, the new SHA-1 of "first" also has prefix badbeef,
then the subsequent 'pick badbeef second' will fail since badbeef is no
longer a unique SHA-1 abbreviation:

  error: short SHA1 badbeef is ambiguous.
  fatal: Needed a single revision
  Invalid commit name: badbeef

Fix this problem by expanding the SHA-1's in the todo list before
performing the operations.

[es: also collapse & expand SHA-1's for --edit-todo; respect
core.commentchar in transform_todo_ids(); compose commit message]

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -689,6 +689,32 @@ skip_unnecessary_picks () {
 	die "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 "$@" ${rest%% *}) &&
+			rest="$sha1 ${rest#* }"
+			;;
+		esac
+		printf '%s\n' "$command${rest:+ }$rest"
+	done <"$todo" >"$todo.new" &&
+	mv -f "$todo.new" "$todo"
+}
+
+expand_todo_ids() {
+	transform_todo_ids
+}
+
+collapse_todo_ids() {
+	transform_todo_ids --short=7
+}
+
 # Rearrange the todo list that has both "pick sha1 msg" and
 # "pick sha1 fixup!/squash! msg" appears in it so that the latter
 # comes immediately after the former, and change "pick" to
@@ -841,6 +867,7 @@ skip)
 edit-todo)
 	git stripspace --strip-comments <"$todo" >"$todo".new
 	mv -f "$todo".new "$todo"
+	collapse_todo_ids
 	append_todo_help
 	git stripspace --comment-lines >>"$todo" <<\EOF
 
@@ -852,6 +879,7 @@ EOF
 
 	git_sequence_editor "$todo" ||
 		die "Could not execute editor"
+	expand_todo_ids
 
 	exit
 	;;
@@ -1008,6 +1036,8 @@ git_sequence_editor "$todo" ||
 has_action "$todo" ||
 	die_abort "Nothing to do"
 
+expand_todo_ids
+
 test -d "$rewritten" || test -n "$force_rebase" || skip_unnecessary_picks
 
 GIT_REFLOG_ACTION="$GIT_REFLOG_ACTION: checkout $onto_name"

t/t3404-rebase-interactive.sh

@@ -1049,7 +1049,7 @@ test_expect_success 'short SHA-1 setup' '
 	)
 '
 
-test_expect_failure 'short SHA-1 collide' '
+test_expect_success 'short SHA-1 collide' '
 	test_when_finished "reset_rebase && git checkout master" &&
 	git checkout collide &&
 	(