log-tree: use decimal_width()

log-tree: use decimal_width()

Reduce code duplication by calling decimal_width() to count the digits
in the number of commits instead of calculating it locally.

It also has the advantage of returning int, which is the exact type
expected by the printf()-like function strbuf_addf() for field width
arguments.

Additionally, decimal_width() supports numbers bigger than 1410065407,
which is (hopefully) just a theoretical advantage.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>