cmd/dist: only use beta tag in version string for the exact revision

Right now, any revision on the default branch after go1.3beta2 is
described by "go verson" as go1.3beta2 plus some revision.
That's OK for now, but once go1.3 is released, that will seem wrong.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/98650046
1 file changed