Rename Commit, Tag to CommitBuilder, TagBuilder

Since these types no longer support reading, calling them a Builder
is a better description of what they do.  They help the caller to
build a commit or a tag object.

Change-Id: I53cae5a800a66ea1721b0fe5e702599df31da05d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>