Use default branch for git and git_subdir resources
Use default branch for git and git_subdir resources

Where git revision is not specified for a dependency, check out and
upgrade from the default branch (origin/HEAD) instead of the hardcoded
`master`, consistently with rebar 2. Mostly to support GitHub repos
where the default branch has been (re)named `main` or something else.

rebar/rebar3,erlang/rebar3,rebar/rebar3