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