time: Duration.String and ParseDuration do not round trip for MinInt64

[dsnet]

Consider this test:

want := time.Duration(math.MinInt64)
got, err := time.ParseDuration(want.String())
if got != want || err != nil {
	t.Errorf("time.ParseDuration = (%v, %v), want (%v, nil)", got, err, want)
}

Unfortunately this test fails:

time.ParseDuration = (0s, time: invalid duration "-2562047h47m16.854775808s"), want (-2562047h47m16.854775808s, nil)

[gopherbot]

Change https://golang.org/cl/352269 mentions this issue: time: allow max negative int64 in ParseDuration

[mknyszek]

CC @rsc via https://dev.golang.org/owners