tg-update.sh: watch out for sneaky blobs
When merging updates throughout the .topdeps graph,
sometimes it's possible to do fast-forward updates.
However, just because a fast-forward update is
possible does not mean it's always correct especially
when it allows .topdeps and/or .topmsg files to sneak
into places where they do not belong.
Avoid this by checking any potential "fast-forards"s
to make sure they satisfy the current top blob constraints
before allowing them to proceed.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>