subtree: add makefile target for html docs

The Makefile currently builds the roff manpage, but not the
html form. As some people may prefer the latter, let's make
it an option to build that, too. We also wire it into "make
doc" so that it is built by default.

This patch does not build or install it as part of
"install-doc"; that would require extra infrastructure to
handle installing the html as we do in git's regular
Documentation/ tree. That can come later if somebody is
interested.

Tested-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/Makefile

@@ -21,13 +21,14 @@ GIT_SUBTREE    := git-subtree
 GIT_SUBTREE_DOC := git-subtree.1
 GIT_SUBTREE_XML := git-subtree.xml
 GIT_SUBTREE_TXT := git-subtree.txt
+GIT_SUBTREE_HTML := git-subtree.html
 
 all: $(GIT_SUBTREE)
 
 $(GIT_SUBTREE): $(GIT_SUBTREE_SH)
 	cp $< $@ && chmod +x $@
 
-doc: $(GIT_SUBTREE_DOC)
+doc: $(GIT_SUBTREE_DOC) $(GIT_SUBTREE_HTML)
 
 install: $(GIT_SUBTREE)
 	$(INSTALL) -d -m 755 $(DESTDIR)$(libexecdir)
@@ -46,6 +47,10 @@ $(GIT_SUBTREE_XML): $(GIT_SUBTREE_TXT)
 	asciidoc -b docbook -d manpage -f $(ASCIIDOC_CONF) \
 		-agit_version=$(gitver) $^
 
+$(GIT_SUBTREE_HTML): $(GIT_SUBTREE_TXT)
+	asciidoc -b xhtml11 -d manpage -f $(ASCIIDOC_CONF) \
+		-agit_version=$(gitver) $^
+
 test:
 	$(MAKE) -C t/ test