The current version was using -style=file option only,
causing failure if pwd != file's directory.
2 kinds of failures occur:
- If no .clang-format is present in PWD, and one is
present in the file's directory, then clang-format
fails for lack of style file, and dump its error message
directly into the buffer
- If a .clang-format is present in PWD, and one is present
in the file's directory, then the wrong .clang-format from PWD is used

This commit makes the behavior fully consistent with clang-format:
- lookup .clang_format in the file's directory and recursively go up
its parent directory until found. If no style file is found, defaults
to g:clang_format#style_options