doc-diff: replace --cut-header-footer with --cut-footer

doc-diff: replace --cut-header-footer with --cut-footer

After the previous commit, AsciiDoc and Asciidoctor render the manpage
headers identically, so we no longer need the "cut the header" part of
our `--cut-header-footer` option. We do still need the "cut the footer"
part, though. The previous commits improved the rendering of the footer
in Asciidoctor by quite a bit, but the two programs still disagree on
how to format the date in the footer: 01/01/1970 vs 1970-01-01.

We could keep using `--cut-header-footer`, but it would be nice if we
had a slightly smaller hammer `--cut-footer` that would be less likely
to hide regressions. Rather than simply adding such an option, let's
also drop `--cut-header-footer`, i.e., rework it to lose the "header"
part of its name and functionality.

`--cut-header-footer` is just a developer tool and it probably has no
more than a handful of users, so we can afford to be aggressive.

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