Float formatting uses a multiprecision fallback path where Grisu3
algorithm fails. This has a bug during the rounding phase: the
difference between the decimal value and the upper bound is examined
byte-by-byte and doesn't properly handle the case where the first
divergence has a difference of 1.

For instance (using an example from #29491), for the number
498484681984085570, roundShortest examines the three decimal values:

lower: 498484681984085536
d:     498484681984085568
upper: 498484681984085600

After examining the 16th digit, we know that rounding d up will fall
within the bounds unless all remaining digits of d are 9 and all
remaining digits of upper are 0:

d:     ...855xx
upper: ...856xx

However, the loop forgets that d and upper have already diverged and
then on the next iteration sees that the 17th digit of d is actually
lower than the 17th digit of upper and decides that we still can't round
up:

d:     ...8556x
upper: ...8560x

Thus the original value is incorrectly rounded down to
498484681984085560 instead of the closer (and equally short)
498484681984085570.

Thanks to Brian Kessler for diagnosing this bug.

Fix it by remembering when we've seen divergence in previous digits.

This CL also fixes another bug in the same loop: for some inputs, the
decimal value d or the lower bound may have fewer digits than the upper
bound, yet the iteration through the digits starts at i=0 for each of
them. For instance, given the float64 value 1e23, we have

d:      99999999999999991611392
upper: 100000000000000000000000

but the loop starts by comparing '9' to '1' rather than '0' to '1'.

I haven't found any cases where this second bug causes incorrect output
because when the digit comparison fails on the first loop iteration the
upper bound always has more nonzero digits (i.e., the expression
'i+1 < upper.nd' is always true).

Fixes #29491

Change-Id: I58856a7a2e47935ec2f233d9f717ef15c78bb2d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/157697
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rémy Oudompheng <remyoudompheng@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>