Set some gc.* git config

In particular, this sets `gc.autoDetach` to "false".

cmd/git-sync/main.go

@@ -408,6 +408,12 @@ func main() {
 		askpassCount.WithLabelValues(metricKeySuccess).Inc()
 	}
 
+	// Set additional configs we want, but users might override.
+	if err := setupDefaultGitConfigs(ctx); err != nil {
+		fmt.Fprintf(os.Stderr, "ERROR: can't set default git configs: %v\n", err)
+		os.Exit(1)
+	}
+
 	// This needs to be after all other git-related config flags.
 	if *flGitConfig != "" {
 		if err := setupExtraGitConfigs(ctx, *flGitConfig); err != nil {
@@ -755,8 +761,8 @@ func addWorktreeAndSwap(ctx context.Context, gitRoot, dest, branch, rev string,
 		return err
 	}
 
-	// GC clone
-	if _, err := cmdRunner.Run(ctx, gitRoot, nil, *flGitCmd, "gc", "--prune=all"); err != nil {
+	// Run GC if needed.
+	if _, err := cmdRunner.Run(ctx, gitRoot, nil, *flGitCmd, "gc", "--auto"); err != nil {
 		return err
 	}
 
@@ -1170,6 +1176,24 @@ func callGitAskPassURL(ctx context.Context, url string) error {
 	return nil
 }
 
+func setupDefaultGitConfigs(ctx context.Context) error {
+	configs := []keyVal{{
+		// Never auto-detach GC runs.
+		key: "gc.autoDetach",
+		val: "false",
+	}, {
+		// Fairly aggressive GC.
+		key: "gc.pruneExpire",
+		val: "now",
+	}}
+	for _, kv := range configs {
+		if _, err := cmdRunner.Run(ctx, "", nil, *flGitCmd, "config", "--global", kv.key, kv.val); err != nil {
+			return fmt.Errorf("error configuring git %q %q: %v", kv.key, kv.val, err)
+		}
+	}
+	return nil
+}
+
 func setupExtraGitConfigs(ctx context.Context, configsFlag string) error {
 	log.V(1).Info("setting additional git configs")