time: fix Parse to ignore extra sub-nanosecond digits
This modifies the code to match the comment such that
the behavior truly is identical to stdSecond case.
Also, it modifies the behavior to match the documented
behavior where:
Fractional seconds are truncated to nanosecond precision.
Fixes #54567
Updates #48685
Change-Id: Ie64549e4372ab51624c105ad8ab4cc99b9b5a0b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/425037
Run-TryBot: Joseph Tsai <joetsai@digital-static.net> Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>