[bitstreams] Default to HEAD for pulling bitstreams

Previously, the default was "latest", but this bitstream may not be part
of the current branch's history. Instead, default to HEAD, and walk
backwards from the current commit.

Signed-off-by: Alexander Williams <awill@google.com>
diff --git a/rules/scripts/bitstreams_workspace.py b/rules/scripts/bitstreams_workspace.py
index f9f6870..9059c95 100755
--- a/rules/scripts/bitstreams_workspace.py
+++ b/rules/scripts/bitstreams_workspace.py
@@ -57,7 +57,7 @@
                     help="Location of the source git repo")
 parser.add_argument(
     'bitstream',
-    default='latest',
+    default='HEAD',
     nargs='?',
     help='Bitstream to retrieve: "latest" or git commit identifier')