Documentation: move callouts.xsl to manpage-{base,normal}.xsl

Documentation: move callouts.xsl to manpage-{base,normal}.xsl

Each of manpage-base.xsl and manpage-normal.xsl gets a copy of
the contents of callouts.xsl and the original is removed. The
Makefile is adjusted to refer to manpage-normal.xsl instead of
callouts.xsl. manpage-base.xsl will be later made into a common
base for -normal and -1.72.

Testing done with asciidoc 8.3.1 and docbook-xsl 1.74.0.

Signed-off-by: Chris Johnsen <chris_johnsen@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>