This commit improves Cargo's support for git repositories whose "main
branch" is not called `master`. Cargo currently pretty liberally assumes
that if nothing else about a git repository is specified then `master`
is the branch name to use. Instead now Cargo has a fourth option as the
desired reference of a repository named `DefaultBranch`. Cargo doesn't
know anything about the actual name of the default branch, it just
updates how git references are fetched internally.
This commit is motivated by news that GitHub is likely to switch away
from the default branch being named `master` in the near future. It
would be a bit of a bummer if from now on everyone had to type
`branch = '...'`, so this tries to improve that!
Only run these tests on one CI builder (not all platforms) though. This
is extremely resource intensive since it rebuilds libstd. Currently this
does not share a build directly like before because the number of tests
are supposed to be small, but if necessary we can add that in later too.
