cmd/go/internal/modfetch/codehost: quote arguments to commands printed from -x

cmd/go/internal/modfetch/codehost: quote arguments to commands printed from -x

Some of the arguments — particularly format strings passed to git commands — may
contain spaces, and it's useful to be able to paste commands from 'go get -x
foo' directly into a shell to reproduce their output.

Change-Id: I4f0c0b4e05db8b5232458e9a271f2ccbb665e85a
Reviewed-on: https://go-review.googlesource.com/126955
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>