The man page of gendiff explains very well how gendiff works, but then it 
makes an example that is not a perfect choice for the place where it's 
being done (a man page):

===
 I.e. if you were
       going to edit somefile.cpp, copy it  to  somefile.cpp.save
       before  editing  it.   Then  edit  the  first  copy (some-
       file.cpp).
 
       After editing all the files you need to edit in this fash-
       ion, enter the directory one level above where your source
       code resides, and then type
           $ gendiff somedirectory .orig > mydiff-orig.patch
===

if you backup your files with .save then gendiff wants .save as extension, 
not .orig.
Since this is a manpage, where new users will look at, it would be better 
to use the backup extension consistently.