twitter: Don't overwrite original contents of retweets

Retweets are apparently still subject to a 140-character limit, and may
be truncated if the original Tweet was an extended Tweet, or if the
additional "RT @user" prefix pushes them beyond the limit.

If `repeatedPostId` has been set by setRepeatedOfInfo(), then `content`
already contains the full text of the original Tweet, and we don't need
to do anything.

(This doesn't apply to TwitterSearch::readStatusesFromJsonMap(), which
does not call setRepeatedOfInfo() in the first place.)

CCBUG:370260