[clang-format] Allow git-clang-format to handle empty extensions.

Most of libc++'s header files don't use extension. This prevents
using git-clang-format on them, which is frustrating.

This patch allows empty extensions to be passed using either
the --extensions option, or the clangformat.extensions git-config
value.

llvm-svn: 305437

clang/tools/clang-format/git-clang-format

@@ -324,6 +324,8 @@ def filter_by_extension(dictionary, allowed_extensions):
   allowed_extensions = frozenset(allowed_extensions)
   for filename in list(dictionary.keys()):
     base_ext = filename.rsplit('.', 1)
+    if len(base_ext) == 1 and '' in allowed_extensions:
+        continue
     if len(base_ext) == 1 or base_ext[1].lower() not in allowed_extensions:
       del dictionary[filename]