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>

Documentation/Makefile

@@ -122,7 +122,8 @@ ASCIIDOC_HTML = xhtml11
 ASCIIDOC_DOCBOOK = docbook
 ASCIIDOC_CONF = -f asciidoc.conf
 ASCIIDOC_COMMON = $(ASCIIDOC) $(ASCIIDOC_EXTRA) $(ASCIIDOC_CONF) \
-		-agit_version=$(GIT_VERSION)
+		-amanversion=$(GIT_VERSION) \
+		-amanmanual='Git Manual' -amansource='Git'
 TXT_TO_HTML = $(ASCIIDOC_COMMON) -b $(ASCIIDOC_HTML)
 TXT_TO_XML = $(ASCIIDOC_COMMON) -b $(ASCIIDOC_DOCBOOK)
 MANPAGE_XSL = manpage-normal.xsl

Documentation/asciidoc.conf

@@ -78,9 +78,9 @@ template::[header-declarations]
 <refmeta>
 <refentrytitle>{mantitle}</refentrytitle>
 <manvolnum>{manvolnum}</manvolnum>
-<refmiscinfo class="source">Git</refmiscinfo>
-<refmiscinfo class="version">{git_version}</refmiscinfo>
-<refmiscinfo class="manual">Git Manual</refmiscinfo>
+<refmiscinfo class="source">{mansource}</refmiscinfo>
+<refmiscinfo class="version">{manversion}</refmiscinfo>
+<refmiscinfo class="manual">{manmanual}</refmiscinfo>
 </refmeta>
 <refnamediv>
   <refname>{manname}</refname>