In an effort to make members of the ProjectCache immutable, this commit makes SubscribeSection an AutoValue. RefSpec is a mutable JGit class that SubscribeSection holds on to. We make a defensive copy when constructing the SubscribeSection and never expose this member. All calls within SubscribeSection are read-only. Change-Id: I2111a1ddf9a1e956cae6152eda170ccccdcc4f5d