Updated dash in front of flags for tar command in fetcher.

Review: https://reviews.apache.org/r/18697/
Changed SIGTERM to SIGKILL in command executor.

Reverting signal from SIGTERM to SIGKILL again per discussion in
https://reviews.apache.org/r/18604/.

Review: https://reviews.apache.org/r/18661
Changed Result::get to return a const reference.

Review: https://reviews.apache.org/r/18434