fix tools/git-sync-deps

Change-Id: I8755fd7a316d284627e25f88e02b69e2f67f9622
Reviewed-on: https://skia-review.googlesource.com/8126
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/tools/git-sync-deps b/tools/git-sync-deps
index 8fec0aa..2b460cb 100755
--- a/tools/git-sync-deps
+++ b/tools/git-sync-deps
@@ -156,7 +156,7 @@
 
   subprocess.check_call([git, 'fetch', '--quiet'], cwd=directory)
 
-  subprocess.check_call([git, 'checkout', '--quiet'], cwd=directory)
+  subprocess.check_call([git, 'checkout', '--quiet', checkoutable], cwd=directory)
 
   if verbose:
     status(directory, checkoutable)  # Success.