Documentation: fix links to tutorials and other new manual pages

Documentation: fix links to tutorials and other new manual pages

With the conversion of HTML documentation to man pages

tutorial.html -> gittutorial (7)
tutorial-2.html -> gittutorial-2 (7)
cvs-migration.html -> gitcvs-migration (7)
diffcore.html -> gitdiffcore (7)
repository-layout.html -> gitrepository-layout (5)
hooks.html -> githooks (5)
glossary.html -> gitglossary (7)
core-tutorial.html -> gitcore-tutorial (7)

and the automatic update of references to these pages,
a little debris was left behind. We clear it away.

Signed-off-by: Jonathan Nieder <jrnieder@uchicago.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>