tighten logging and print an informational header

[tbg]

This PR tightens up logging. In particular, it makes sure the spinner lines are completely gone when completed. Also, we print (to stdout) a header summarizing the old and new sha as well as the benchdiff invocation. The idea is that these are always useful to include with the results.

Example output:

$ benchdiff -c 1 -d 1x .
old:  a57c216 Merge pull request #34 from tbg/col-prof
new:  0df532a Merge pull request #36 from tbg/main
args: benchdiff "-c" "1" "-d" "1x" "."

name                                                        old time/op    new time/op    delta
RPC/payloadSize=______1/framework=grpc/mode=Unary-10           848µs ± 0%     935µs ± 0%   ~     (p=1.000 n=1+1)
RPC/payloadSize=______1/framework=grpc/mode=Stream-10          217µs ± 0%     156µs ± 0%   ~     (p=1.000 n=1+1)
[...]