Make refs/remotes/m management the manifest object's responsibility

I plan to have the new submodule manifest format use a different
layout for the m refs than the XML manifest format has used in
the past.  Thus we need to move the behavior management into the
manifest object, and out of the project, so we can change it.

Signed-off-by: Shawn O. Pearce <sop@google.com>
4 files changed