repo/init: improve basic progress messages

We produce some simple "Get" messages that aren't super clear as to
what they're doing, especially for people not familiar with repo.
Rephrase these to explicitly state the thing we're doing so it's
clear why we're downloading a particular source.

Bug: https://crbug.com/gerrit/11293
Change-Id: I0749504f17c5385c6c65274a274e0ae25b117413
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256455
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>