init: support -b HEAD as a shortcut to "the default"

When people switch to non-default branches, they sometimes want to
switch back to the default, but don't know the exact name for that
branch.  Add a -b HEAD shortcut for that.

Change-Id: I090230da25f9f5a169608115d483f660f555624f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297843


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>