text/tabwriter: don't mimic previous lines on flush

text/tabwriter: don't mimic previous lines on flush

\f triggers a flush.

This is used (by gofmt, among others) to indicate that
the current aligned segment has ended.

When flushed, it is unlikely that the previous line is
in fact a good predictor of the upcoming line,
so stop treating it as such.

No performance impact on the existing benchmarks,
which do not perform any flushes.

Change-Id: Ifdf3e6d4600713c90db7b51a10e429d9260dc08c
Reviewed-on: https://go-review.googlesource.com/111644
Reviewed-by: Robert Griesemer <gri@golang.org>