When creating the refs for the package base branch and the HEAD ref
workaround, directly use commit IDs instead of symrefs. It looks like
Git cannot pack symrefs, so this is expected to improve space
efficiency. Also, since the refs are now only created (or updated) when
new objects are actually received, this fixes a problem with dangling
refs that were created when a user initialized a repository but did not
push anything.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>