go.tools/cmd/benchcmp: do not generate trailing whitespace
go.tools/cmd/benchcmp: do not generate trailing whitespace

text/tabwriter cells are tab-terminated, not tab-separated. This creates trailing whitespace. However, with left-aligned columns, it is ok not to treat the final element as a cell. Demo: http://play.golang.org/p/m_ajG8SSZe

LGTM=gri
R=golang-codereviews, gobot, gri
CC=golang-codereviews, r
https://codereview.appspot.com/103650043

Committer: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools