repo: avoid bare excepts to allow SystemExit to bubble

Bug: https://crbug.com/gerrit/12327
Change-Id: I4ce1142379b111f9ba3a2e5a437026e5c0378a9e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254756
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>