Add support for a pdf version of the user manual

Add support for a pdf version of the user manual

Use dblatex in order to create a pdf version of the git user manual.  No
existing Makefile targets (including "all") are touched, so you need to
explicitly say

make pdf
sudo make install-pdf

to get user-manual.pdf created and installed.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>