rebase: add --fail-fast support

Lets switch the default rebase behavior to align with our new sync
behavior: we try to rebase all projects by default and exit/summarize
things at the very end if there were any errors.  Or if people want
to exit immediately, they can use the new --fail-fast option.

Change-Id: I436ac563f972b45de6ce9ad74da1e4870e584902
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/238553
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>