transport: allow summary-width to be computed dynamically

transport: allow summary-width to be computed dynamically

Now we have identified three callchains that have a set of refs that
they want to show their <old, new> object names in an aligned output,
we can replace their reference to the constant TRANSPORT_SUMMARY_WIDTH
with a helper function call to transport_summary_width() that takes
the set of ref as a parameter.  This step does not yet iterate over
the refs and compute, which is left as an exercise to the readers.

Signed-off-by: Junio C Hamano <gitster@pobox.com>