contrib/subtree/Makefile: clean up rules to generate documentation

contrib/subtree/Makefile: clean up rules to generate documentation

git:Documentation/Makefile establishes asciidoc/xmlto calls as being
handled through their appropriate variables, Hence, change to bring into
congruency with.

Similarly, MANPAGE_XSL exists in git:Documentation/Makefile, while
MANPAGE_NORMAL_XSL does not outside contrib/subtree. Hence, replace
MANPAGE_NORMAL_XSL with MANPAGE_XSL.

Reviewed-by: Jeff King <redacted>
Signed-off-by: James Denholm <redacted>
Signed-off-by: Junio C Hamano <redacted>