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>

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