Documentation: convert tutorials to man pages

Documentation: convert tutorials to man pages

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

cvs-migration.txt -> gitcvs-migration.txt
tutorial.txt      -> gittutorial.txt
tutorial-2.txt    -> gittutorial-2.txt

These new man pages are put in section 7, and other documents that reference
the above ones are change accordingly.

[jc: with help from Nanako to clean things up]

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