When a user hits "Reply", then "Cancel" on an inline comment (without typing), don't show "Undo"

Summary: When a user hits "Reply" on an inline comment, doesn't type anything,
and then hits "Cancel", we incorrectly store the text of the comment the user is
replying to as the "original" text, and then detect that they've changed it when
they immediately cancel. Instead, store empty string as the original text.

Test Plan:

• Hit "Reply" and then "Cancel" on an inline comment. No undo now.
• Hit "Reply", typed some text, and then hit "Cancel". Got an undo which

restored my text.

Reviewers: tomo, jungejason, tuomaspelkonen, aran

Reviewed By: aran

CC: aran, tomo

Differential Revision: 879