Add an GIT_HEAD_IS_DEFAULT_BRANCH variable.

[jmalloc]

The default branch can be obtained reliably by querying the origin server using:

git remote show origin | grep "  HEAD branch:" | cut -c16-

We could check locally first by querying the HEAD of origin (not the HEAD of any particular branch, but the origin remote itself):

git symbolic-ref refs/remotes/origin/HEAD

However, this can be unavailable, or get out of sync if the default branch is changed after the clone is made. That's not a big concern for CI, but it is for long-lived clones on development workstations.

git remote set-head origin --auto

The command above can be used to sync the ref locally, but that obviously has to contact the server anyway, so is no faster than the git remote show approach.

As an aside, it seems that grit does not populate the remote head locally when cloning.