fix: skip git tarball when necessary

It's possible to run cargo-dist in something that's not a git repo; we should
skip generating a tarball automatically in that case.

cargo-dist/src/tasks.rs

@@ -1201,6 +1201,16 @@ impl<'pkg_graph> DistGraphBuilder<'pkg_graph> {
         if !self.global_artifacts_enabled() {
             return;
         }
+
+        // It's possible to run cargo-dist in something that's not a git
+        // repo, including a brand-new repo that hasn't been `git init`ted yet;
+        // we can't act on those.
+        let root_repo = self.inner.workspace_dir.join(".git");
+        if !root_repo.exists() {
+            warn!("skipping source tarball; no git repo found at {root_repo}");
+            return;
+        }
+
         let release = self.release(to_release);
         let checksum = release.checksum;
         info!("adding source tarball to release {}", release.id);