Use consistent argv-name in help-message and code

* I wasted some time because fetch-wt.js fetched the wrong
  rev-id because I used this command-line based on looking at
  the optimist arg-key name (without looking at the code).

$ fetch-wt.js --prefix fr --title Villes_aux_États-Unis --rev 94897581

  It just fetched the latest WT for that title and I was seeing
  some odd selser diffs in my testing as a result.

* Other minor tweaks.

Change-Id: Ib4b197a74360942af3aea745bed7769a8338a6f3
Merge "Use consistent argv-name in help-message and code"