Documentation: complete config list from other manpages

Documentation: complete config list from other manpages

Add an autogeneration script Documentation/make-config-list.perl
that complete list of config variables with missing variables
from other manpages.

This script generates minimal documentation for those missing config
variables at appropriate place in Documentation/config-vars.txt,
using the following form:

    foo.bar::
    foo.baz::
        See linkgit:git-foo[1].

It does that as follows:

* parse config-vars-src.txt (was config-vars.txt, which is now generated)
  to find out config variables it contains

* parse each manpage source (following includes) for config variable
  headers

* assemble a new config-vars.txt that completes the original list with
  "See linkgit:git-foo[1]" entries for all variables that were not in
  it config-vars-src.txt

Because config-vars.txt is now generated, and as a result can change
if any of examined files change, there was added automatically
generated dependency for this file (using the same script, but with
'--deps' option).

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>