diff: handle errors gracefully

If `git diff` fails in any project checkout (e.g. an incomplete
sync), make sure we print that error clearly rather than blowing
up, and exit non-zero in the process.

Bug: https://crbug.com/gerrit/11613
Change-Id: I12f278427cced20f23f8047e7e3dba8f442ee25e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239236
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>