cmd/go: add join template function.
It's common to use the go list command in shell scripts, but
currently it's awkward to print a string slice from the Package
type in a way that's easily parseable by the shell. For example:
go list -f '{{range .Deps}}{{.}}
{{end}}'
(and even that prints an unwanted new line at the end|).
To make this easier, this CL adds a "join" function to the
format template.
go list -f '{{join .Deps "\n"}}'
R=rsc, dsymonds, minux.ma, remyoudompheng, r
CC=golang-dev
https://golang.org/cl/
6680044