Fix tag clobbering when -c is used.

Bug: b/140189154
Change-Id: I8861a6115b20c9a3d88ddec5344c75326ae44823
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/237572


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Xin Li <delphij@google.com>