launcher: change RunResult to subprocess.CompletedProcess

Since we require Python 3.6 now in the launcher, swap out our custom
RunResult class for the standard subprocess one.

Change-Id: Idd8598df37c0a952d3ef828df6e250cab03c6589
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/462341


Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>