hgignore: adds bin/ to support setting $GOBIN to $GOROOT/bin
hgignore: adds bin/ to support setting $GOBIN to $GOROOT/bin

R=PeterGo, adg
CC=golang-dev, jnw
http://codereview.appspot.com/1699046

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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