asciidoctor-extensions.rb.in: inject GIT_DATE

asciidoctor-extensions.rb.in: inject GIT_DATE

After a38edab7c8 (Makefile: generate doc versions via GIT-VERSION-GEN,
2024-12-06), we no longer inject GIT_DATE when building with
Asciidoctor.

Replace the <date/> tag in the XML to inject the value of GIT_DATE.
Unlike <refmiscinfo/> as handled in a recent commit, we have no reason
to expect that this tag might be missing, so there's no need for "maybe
remove, then add" and we can just outright replace the one that
Asciidoctor has generated based on the mtime of the source file.

Compared to pre-a38edab7c8, we now end up injecting this also in the
build of Git.3pm, which until now has been using the mtime of Git.pm.
That is arguably even a good change since it results in more
reproducible builds.

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