This reduces the number of allocs when
running the rotate.go tests by
about 20%, after applying CL 5700.

Combining

s = "const str"
s += <another string>

generally saves an alloc and might be a candidate for
rsc's grind tool. However, I'm sending this CL now
because this also reuses the result of calling lexbuf.String.

Change-Id: If3a7300b7da9612ab62bb910ee90349dca88dde3
Reviewed-on: https://go-review.googlesource.com/5821Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>