doc/Makefile: drop doc-diff worktree and temporary files on "make clean"

doc-diff creates a temporary working tree (git-worktree) and generates a
bunch of temporary files which it does not remove since they act as a
cache to speed up subsequent runs. Although doc-diff's working tree and
generated files are not strictly build products of the Makefile (which,
itself, never runs doc-diff), as a convenience, update "make clean" to
clean up doc-diff's working tree and generated files along with other
development detritus normally removed by "make clean".

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -331,6 +331,7 @@ clean:
 	$(RM) SubmittingPatches.txt
 	$(RM) $(cmds_txt) $(mergetools_txt) *.made
 	$(RM) manpage-base-url.xsl
+	'$(SHELL_PATH_SQ)' ./doc-diff --clean
 
 $(MAN_HTML): %.html : %.txt asciidoc.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \