[PATCH] Fix Info documentation build with Docbook2X 0.8.8

Hi,

The attached patch against 1.5.4.4 (from NixOS/Nixpkgs) fixes build of
the user manual in Info (i.e., `git.info') through Docbook2X 0.8.8.
Actually, it does two things:

  1. Fix the value of `DOCBOOK2TEXI' to match the name currently used in
     Docbook2X (I don't know when that name change occurred).

  2. Disable build of `gitman.info', which cannot be built because of
     duplicate node names:

     $ makeinfo gitman.texi
     gitman.texi:273: Node `DESCRIPTION' previously defined at line 153.
     gitman.texi:281: Node `OPTIONS' previously defined at line 163.
     gitman.texi:360: Node `DISCUSSION' previously defined at line 205.
     [...]

A related patch against Docbook2X 0.8.8 is needed to make `--to-stdout'
work as expected [0].

Thanks,
Ludovic.

[0] http://sourceforge.net/tracker/index.php?func=detail&aid=1912754&group_id=7856&atid=107856