hghave: cache the result of gethgversion
hghave --test-features calls it 90 times, each one calling hg --version
which takes a tenth of a second on my workstation, adding up to about
10s win on test-hghave.t.
Fixes https://bugs.debian.org/939756
Differential Revision: https://phab.mercurial-scm.org/D8092
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.