This patches enables to perform textconv with blame if a textconv driver is
available fos the file.
The main task is performed by the textconv_object function which prepares
diff_filespec and if possible converts the file using diff textconv API.
Only regular files are converted, so the mode of diff_filespec is faked.
Textconv conversion is enabled by default (equivalent to the option
--textconv), since blaming binary files is useless in most cases.
The option --no-textconv is used to disable textconv conversion.
The declarations of several functions are modified to give access to a
diff_options, in order to know whether the textconv option is activated or not.