dd2bd5c adds a check in the commit message edit service to
prevent a new patch set from being added when the given commit
message is the same as the existing one. However it is still
possible for the user to send a commit edit request from the Web
UI with an unchanged commit message, which results in an error
dialog.
Add a new helper class that watches for changes on a text box
and notifies changes. Use this in the commit message editor
dialog to enable the send button only when the text differs from
the existing commit message text.
Change-Id: I249beaa24e8a30bddd6f6a471053396a3fd2dafa
7ca14ba346