This commit follows through with work started in rust-lang#8522 to change the
default behavior of `git` dependencies where if not branch/tag/etc is
listed then `HEAD` is used instead of the `master` branch. This involves
also changing the default lock file format, now including a `version`
marker at the top of the file notably as well as changing the encoding
of `branch=master` directives in `Cargo.toml`.

If we did all our work correctly then this will be a seamless change.
First released on stable in 1.47.0 (2020-10-08) Cargo has been emitting
warnings about situations which may break in the future. This means that
if you don't specify `branch = 'master'` but your HEAD branch isn't
`master`, you've been getting a warning. Similarly if your dependency
graph used both `branch = 'master'` as well as specifying nothing, you
were receiving warnings as well. These two situations are broken by this
commit, but it's hoped that by giving enough times with warnings we
don't actually break anyone in practice.

This commit fixes a bug in Cargo where after `DefaultBranch` and
`Branch("master")` are considered distinct no v2 lockfile would by
default match a dependency specification with the `branch = 'master'`
annotation. A feature of the v2 lockfile, however, is that no mention of
a branch is equivalent to the `master` branch.

The bug here is fixed by updating the code which registers a previous
lock file with our `PackageRegistry`. This code registers nodes, only
with v2 lock files, for both default and `master` branch git
dependencies. This way requests for either one will get matched now that
they're considered distinct.