Subzero: Change szbuild.py to prefer Subzero by default.

Subzero translation is stable enough that szbuild.py should prefer Subzero-translated symbols by default.  The exception is that if you explicitly use --include, the intuitive interpretation is that you only want Subzero to include those symbols (minus any given with --exclude).

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/605283002
diff --git a/pydir/szbuild.py b/pydir/szbuild.py
index ab67f02..63d7db0 100755
--- a/pydir/szbuild.py
+++ b/pydir/szbuild.py
@@ -219,9 +219,9 @@
     re_include = re.compile(re_include_str)
     re_exclude = re.compile(re_exclude_str)
     # If a symbol doesn't explicitly match re_include or re_exclude,
-    # the default MatchSymbol() result is False, unless some --exclude
-    # args are provided and no --include args are provided.
-    default_match = len(args.exclude) and not len(args.include)
+    # the default MatchSymbol() result is True, unless some --include
+    # args are provided.
+    default_match = not len(args.include)
 
     whitelist_has_items = False
     with open(whitelist_sz_unescaped, 'w') as f: