doc-diff: support diffing from/to AsciiDoc(tor)

doc-diff: support diffing from/to AsciiDoc(tor)

Provide `--from-asciidoctor` and `--to-asciidoctor` to select that the
"from" resp. "to" commit should be built with Asciidoctor, and provide
an `--asciidoctor` shortcut for giving both. Similarly, provide
--{from-,to-,}asciidoc for explicitly selecting AsciiDoc.

Implement this using the USE_ASCIIDOCTOR flag. Let's not enforce a
default here, but instead just let the Makefile fall back on whatever is
in config.mak, so that `./doc-diff foo bar` without any of of these new
options behaves exactly like it did before this commit.

Encode the choice into the directory names of our "installed" and
"rendered" files, so that we can run `./doc-diff --from-asciidoc
--to-asciidoctor HEAD HEAD` without our two runs stomping on each other.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>