[git-clang-format] recognize hxx as a C++ file

clangd, clang-tidy, etc does that already, no reason why
git-clang-format should skip hxx files.

Reviewed By: ilya-biryukov

Differential Revision: https://reviews.llvm.org/D63621

llvm-svn: 364014

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

@@ -77,7 +77,7 @@ def main():
       'c', 'h',  # C
       'm',  # ObjC
       'mm',  # ObjC++
-      'cc', 'cp', 'cpp', 'c++', 'cxx', 'hpp',  # C++
+      'cc', 'cp', 'cpp', 'c++', 'cxx', 'hpp', 'hxx',  # C++
       'cu',  # CUDA
       # Other languages that clang-format supports
       'proto', 'protodevel',  # Protocol Buffers