x/tools/gopls: address formatting issues with go-diff@v1.2.0

[findleyr]

In #45648, we discovered that gopls has formatting bugs when built with github.com/sergi/go-diff@v1.2.0. Unfortunately, there are probably many users that get this version due to installing with go get -u.

Until sergi/go-diff#115 is fixed, or we find some other solution, we should at least warn users when built with go-diff@v1.2.0.

CC @stamblerre @heschi

[gopherbot]

Change https://golang.org/cl/315852 mentions this issue: internal/lsp: warn users who have built gopls with go-diff v1.2.0