tg: awksome accelerate top-bases computation
Make use of the new run_awk_ref_prefixes function to compute
the active top-bases prefix for the repository or the remote
being populated.
Although only performed once at tg startup (or remote --populate
time), using the awksome function makes the overhead for the
computation practically nil. It also brings the side benefit
of ignoring orphan bases (bases with no corresponding head) to
the remote --populate operation. A win all around.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>