Quote ' as \(aq in manpages

Quote ' as \(aq in manpages

The docbook/xmlto toolchain insists on quoting ' as \'.  This does
achieve the quoting goal, but modern 'man' implementations turn the
apostrophe into a unicode "proper" apostrophe (given the right
circumstances), breaking code examples in many of our manpages.

Quote them as \(aq instead, which is an "apostrophe quote" as per the
groff_char manpage.

Unfortunately, as Anders Kaseorg kindly pointed out, this is not
portable beyond groff, so we add an extra Makefile variable GNU_ROFF
which you need to enable to get the new quoting.

Thanks also to Miklos Vajna for documentation.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>