Documentation: convert "glossary" and "core-tutorial" to man pages

Documentation: convert "glossary" and "core-tutorial" to man pages

This patch renames the following documents and at the same time converts
them to the man format:

core-tutorial.txt -> gitcore-tutorial.txt
glossary.txt      -> gitglossary.txt

But as the glossary is included in the user manual and as the new
gitglossary man page cannot be included as a whole in the user manual,
the actual glossary content is now in its own "glossary-content.txt"
new file. And this file is included by both the user manual and the
gitglossary man page.

Other documents that reference the above ones are changed accordingly
and sometimes improved a little too.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>