Documentation/Makefile: break up texi pipeline

Documentation/Makefile: break up texi pipeline

Most shells define the exit value of a pipeline as the exit value
of the last process. For each texi rule, run the DOCBOOK2X_TEXI
tool and the "fixup" script in their own non-pipeline commands so
that make will notice an error exit code.

Signed-off-by: Chris Johnsen <chris_johnsen@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>