Make Commit class only for writing

The Commit class now only supports the creation of a commit object.
To read a commit, applictions should use RevCommit.  This permits
us to have exactly one implementation, and RevCommit's is faster
and more bug-free.

Change-Id: Ib573f7e15f36855112815269385c21dea532e2cf
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>