Explicitly allow clobbering tags when fetching from remote.

Bug: b/139860049
Change-Id: I3c4134eda7e9e75c9d72b233e269bcc0e624d1e8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/234632
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Xin Li <delphij@google.com>