doc: simplify man version

doc: simplify man version

The hacks to add version information to the man pages comes from 2007
7ef195ba3e (Documentation: Add version information to man pages,
2007-03-25). In that code we passed three fields to DocBook Stylesheets:
`source`, `version`, and `manual`, however, all the stylesheets do is
join the strings `source` and `version` [1].

Their own documentation explains that in pracice the source is just a
combination of two fields [2]:

  In practice, there are many pages that simply have a version number in
  the "source" field.

Splitting that information might have seemed more proper in 2007, but it
not achieve anything in practice.

Asciidoctor had support for this information in their manpage backend
since day 1: v1.5.3 (2015), but it didn't include the version. In the
docbook5 backend they did in v1.5.7 (2018), but again: no version.

There is no need for us to demand that that they add support for the
version field when in reality all that is going to happen is that both
fields are going to be joined.

Let's do that ourselves so we can forget about all our hacks for this
and so it works for both asciidoc.py, and docbook5 and manpage backends
of asciidoctor.

[1] https://github.com/docbook/xslt10-stylesheets/blob/master/xsl/common/refentry.xsl#L545
[2] https://docbook.sourceforge.net/release/xsl/current/doc/common/template.get.refentry.source.html

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>