This makes the .texi basename the same as the package name,
which is consistent with most other GNU packages.
* doc/diffutils.texi: Rename from...
* doc/diff.texi: ...removed.
* doc/Makefile.am (info_TEXINFOS): Reflect name change.
(diffutils_TEXINFOS): Likewise.
* cfg.mk (gendocs_options_): Remove this customization. It is no
longer needed, now that the .texi name matches that of the package.
* man/Makefile.am ($(dist_man1_MANS)): Remove now-unnecessary sed
filter.
* README: Update references to diffutils.texi etc. here, too.
* .gitignore: Update here, too.
Suggested by Karl Berry.