asciidoctor: fix user-manual to be built by `asciidoctor`

The `user-manual.txt` is designed as a `book` but the `Makefile` wants
to build it as an `article`. This seems to be a problem when building
the documentation with `asciidoctor`. Furthermore the parts *Git
Glossary* and *Appendix B* had no subsections which is not allowed when
building with `asciidoctor`. So lets add a *dummy* section.

Signed-off-by: 마누엘 <nalla@hamal.uberspace.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -337,7 +337,7 @@ manpage-base-url.xsl: manpage-base-url.xsl.in
 
 user-manual.xml: user-manual.txt user-manual.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(TXT_TO_XML) -d article -o $@+ $< && \
+	$(TXT_TO_XML) -d book -o $@+ $< && \
 	mv $@+ $@
 
 technical/api-index.txt: technical/api-index-skel.txt \

Documentation/user-manual.txt

@@ -4395,6 +4395,10 @@ itself!
 Git Glossary
 ============
 
+[[git-explained]]
+Git explained
+-------------
+
 include::glossary-content.txt[]
 
 [[git-quick-start]]
@@ -4636,6 +4640,10 @@ $ git gc
 Appendix B: Notes and todo list for this manual
 ===============================================
 
+[[todo-list]]
+Todo list
+---------
+
 This is a work in progress.
 
 The basic requirements: