git: do not export new ref if racy process created one with the same name

Since we've checked the ref doesn't exist in this code path, I think it's
better to not overwrite the existing ref.

lib/src/git.rs

@@ -603,7 +603,7 @@ fn update_git_ref(
             } else {
                 // The branch was added in jj but still doesn't exist in git, so add it
                 git_repo
-                    .reference(git_ref_name, new_oid, true, "export from jj")
+                    .reference(git_ref_name, new_oid, false, "export from jj")
                     .map_err(FailedRefExportReason::FailedToSet)?;
             }
         }