make lineno_width() from blame reusable for others
builtin/blame.c has a helper function to compute how many columns
we need to show a line-number, whose implementation is reusable as
a more generic helper function to count the number of columns
necessary to show any cardinal number.
Rename it to decimal_width(), move it to pager.c and export it for
use by future callers.
Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl> Signed-off-by: Junio C Hamano <gitster@pobox.com>