command: Add parentheses on wrapped condition

Surround the condition with parentheses rather than using
backslashes. This prevents confusion about indentation when
running flake8/autoflake8.

Change-Id: I01775b96f817ee616f545b55369a4864fa1d6712
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254603
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: David Pursehouse <dpursehouse@collab.net>