Ignore autogenerated change messages while linking comments to them

This change is needed after we moved the implementation of linking
comments to change messages to the backend (previously the matching was
done in the front end using time stamps matching).

Linking with timestamps used to work generally fine: We assign each
comment to the nearest next change message, w.r.t. timestamps. After a
recent change (see I3f642255c248f), we discovered that the use case of
publishing comments with git push results in 2 change messages with the
same timestamp: the patch set upload change message, and the published
comments message, making the assignment of comments to the correct
message non-deterministic (using the timestamps matching).

In this change we are using the property that the patch set upload
change message is always tagged as "autogenerated", hence we are simply
ignoring it, resulting in a deterministic timestamp matching with the
correct change message.

Change-Id: Id7efe53351738f5c6181501777b5921eb72a6a54