internal/stdlib: pass -f to git fetch in remoteGoRepo.clone

remoteGoRepo.clone inits a repo and then tries to fetch into the
"main" branch of the repo. But if the user's settings are for the
default branch to be called main, main will already exist and the
fetch command will fail. The -f flag will cause the command to do the
fetch even though the branch already exists.

For golang/go#63254
For golang/go#63744

Change-Id: I33f414654088a67eae4bfa70424d0dc6c34903f0
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/540635
LUCI-TryBot-Result: Go LUCI <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
kokoro-CI: kokoro <[email protected]>
Run-TryBot: Michael Matloob <[email protected]>

Author: matloob

internal/stdlib/gorepo.go

@@ -44,7 +44,7 @@ func (remoteGoRepo) clone(ctx context.Context, v, directory string) (refName str
 	if err := cmd.Run(); err != nil {
 		return "", err
 	}
-	cmd = exec.CommandContext(ctx, "git", "fetch", "--depth=1", "--", GoRepoURL, refName+":main")
+	cmd = exec.CommandContext(ctx, "git", "fetch", "-f", "--depth=1", "--", GoRepoURL, refName+":main")
 	cmd.Dir = directory
 	if b, err := cmd.CombinedOutput(); err != nil {
 		return "", fmt.Errorf("running git fetch: %v: %s", err, b)