Pass --update-head-ok when fetching via git CLI

Discovered in a recent [comment] it looks like not passing this may
cause the git CLI to fail in some situations.

[comment]: #2078 (comment)

src/cargo/sources/git/utils.rs

@@ -755,6 +755,7 @@ fn fetch_with_cli(
  cmd.arg("fetch")
  .arg("--tags") // fetch all tags
  .arg("--quiet")
+ .arg("--update-head-ok") // see discussion in #2078
  .arg(url.to_string())
  .arg(refspec)
  .cwd(repo.path());