Doc/Makefile: give mansource/-version/-manual attributes

Rather than hardcoding "Git Manual" and "Git" as the manual and source in asciidoc.conf, provide them as attributes `manmanual` and `mansource`. Rename the `git_version` attribute to `manversion`. These new attribute names are not arbitrary, see, e.g., [1]. For AsciiDoc (8.6.10) and Asciidoctor <1.5.7, this is a no-op. Starting with Asciidoctor 1.5.7, `manmanual` and `mansource` actually end up in the xml-files and eventually in the rendered manpages. In particular, the manpage headers now render just as with AsciiDoc. No versions of Asciidoctor pick up the `manversion` [2], and older versions don't pick up any of these attributes. -- We'll fix that with a bit of a hack in the next commit. [1] https://asciidoctor.org/docs/user-manual/#man-pages [2] Note how [1] says "Not used by Asciidoctor". Signed-off-by: Martin Ågren <martin.agren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>