cros_build_lib: convert CommandResult to CompletedProcess

Since CommandResult is just CompletedProcess at this point, switch
away from that to better match subprocess APIs.

BUG=b:187789262
TEST=CQ passes

Change-Id: I888ef8c27d8a57b6461deade4ff57846c6e8b04e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3818209
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
diff --git a/scripts/autotest_quickmerge.py b/scripts/autotest_quickmerge.py
index 7b0cc50..4fb59d5 100644
--- a/scripts/autotest_quickmerge.py
+++ b/scripts/autotest_quickmerge.py
@@ -254,7 +254,7 @@
                not just older files.
 
   Returns:
-    The cros_build_lib.CommandResult object resulting from the rsync command.
+    The cros_build_lib.CompletedProcess object resulting from the rsync command.
   """
   command = ['rsync', '-a']