flag: use strings.Builder instead of concatenating strings

[jamespfennell]

There is a single function in the flag package whose implementation uses string concatenation instead of the recommended strings.Builder. The function was last touched before strings.Builder was introduced in Go 1.10, which explains the old style code.

I am about to propose a PR to migrate the function to strings.Builder; filing this for tracking purposes.

[gopherbot]

Change https://golang.org/cl/307329 mentions this issue: flag: use strings.Builder instead of concatenating strings

[ianlancetaylor]

Note that it's not necessary to file an issue for a change that does not affect anything user visible. It's OK to just send the change. But filing an issue is OK too. Thanks.