ci: Improve log output (mainly Travis).

* Bring back colors on Travis, which was disabled since #39036.
  Append --color=always to cargo when running in CI environment.
* Removed `set -x` in the shell scripts. The `retry` function already
  prints which command it is running, add `-x` just add noise to the
  output.
* Support travis_fold/travis_time. Matching pairs of these allow Travis CI
  to collapse the output in between. This greatly cut down the unnecessary
  "successful" output one need to scroll through before finding the failed
  statement.

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