Make style lookup consistent with clang-format

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

Author: arntanguy

autoload/clang_format.vim

@@ -148,15 +148,16 @@ let g:clang_format#auto_formatexpr = s:getg('clang_format#auto_formatexpr', 0)
 function! s:detect_style_file()
     let dirname = expand('%:p:h')
     let style_file_name = has('win32') || has('win64') ? '_clang-format' : '.clang-format'
-    return findfile(style_file_name, dirname.';') != ''
+    return findfile(style_file_name, dirname.';')
 endfunction
 
 function! clang_format#format(line1, line2)
     let args = printf(" -lines=%d:%d", a:line1, a:line2)
-    if ! (g:clang_format#detect_style_file && s:detect_style_file())
+    let style_file = s:detect_style_file()
+    if ! (g:clang_format#detect_style_file && style_file != '')
         let args .= printf(" -style=%s ", s:make_style_options())
     else
-        let args .= " -style=file "
+        let args .= " -style=file --assume-filename=".style_file
     endif
     let args .= g:clang_format#extra_args
     let clang_format = printf("%s %s --", g:clang_format#command, args)