forall: add an --ignore-missing option

In CrOS, our infra has to deal with partial checkouts constantly
(for a variety of reasons).  To help reset back to a good state,
we run git commands via `repo forall`, but don't care about the
missing checkouts.  Add a flag so we can disambiguate between
missing repos and failing git subcommands.

Bug: https://crbug.com/1013377
Bug: https://crbug.com/1013623
Change-Id: Ie3498c6d111276c60d2ecedbba21bfa778588d50
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/241935
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>