Add a "gendiff" backend for %autosetup

This is just plain patch, but generates backups suffixed by patch
ordinal (NOT patch number) for the benefit of gendiff. Hence the name.

Thanks to Per Øyvind Karlsen and Neal Gompa for inspiration, input
and patch suggestions in PR#109 and PR#110.

macros.in

@@ -1089,6 +1089,11 @@ package or when debugging this package.\
 %__scm_apply_patch(qp:m:)\
 %{__patch} %{-p:-p%{-p*}} %{-q:-s} --fuzz=%{_default_patch_fuzz} %{_default_patch_flags}
 
+# Plain patch with backups for gendiff
+%__scm_setup_gendiff(q) %{nil}
+%__scm_apply_gendiff(qp:m:)\
+%{__patch} %{-p:-p%{-p*}} %{-q:-s} --fuzz=%{_default_patch_fuzz} %{_default_patch_flags} -b --suffix ".%{2}"
+
 # Mercurial (aka hg)
 %__scm_setup_hg(q)\
 %{__hg} init %{-q} .\