Support parsing previous checkout as a revision expresion.

Support parsing previous checkout as a revision expresion.

Repository.resolve can only return an ObjectId and will
continue to do so, but another method, simplify(), will
be able to return a branch name for some cases.

Previous checkouts can be specified as @{-n}, where n is an
integer speifying the n:th previous branch. The result
is the branch name, unless the checkout was a detached head,
in which case the object id is returned. Since the result
is a branch it may be followed by a references to the reflog,
such as @{-1}@{1} if necessary.

A simple expression like "master" is resolved to master in
simplify, but anything starting with refs gets resolved to
its object id, even if it is a branch.

A symbolic ref is resolved to its leaf ref, e.g. "HEAD" might
be resolved to "master".

Change-Id: Ifb815a1247ba2a3e2d9c46249c09be9d47f2b693