internal/lsp/cmd: use 'go-diff' by default in format command
internal/lsp/cmd: use 'go-diff' by default in format command

Change-Id: Idf6c828721921df21de3d25930580ad5ed5808d3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/202620
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools