doc/Makefile: do not include hg.1.gendoc.txt in MANIFEST

It is an intermediate file used to produce the hg.1 and .hg.1.html
files and is not useful for people who download the tarball. It will
be regenerated automatically by the Makefile if users want to rebuild
the documentation.

doc/Makefile

@@ -34,7 +34,7 b' hg.1.gendoc.txt: $(GENDOC)'
 MANIFEST: man html
 # tracked files are already in the main MANIFEST
 	$(RM) $@
-	for i in $(MAN) $(HTML) hg.1.gendoc.txt; do \
+	for i in $(MAN) $(HTML); do \
 	  echo "doc/$$i" >> $@ ; \
 	done