Add support for an info version of the user manual

Add support for an info version of the user manual

These patches use docbook2x in order to create an info version of the
git user manual.  No existing Makefile targets (including "all") are
touched, so you need to explicitly say

make info
sudo make install-info

to get git.info created and installed.  If the info target directory
does not already contain a "dir" file, no directory entry is created.
This facilitates $(DESTDIR)-based installations.  The same could be
achieved with

sudo make INSTALL_INFO=: install-info

explicitly.

perl is used for patching up sub-par file and directory information in
the Texinfo file.  It would be cleaner to place the respective info
straight into user-manual.txt or the conversion configurations, but I
find myself unable to find out how to do this with Asciidoc/Texinfo.

Signed-off-by: David Kastrup <dak@gnu.org>