Documentation: stop depending on Perl to massage user manual

Documentation: stop depending on Perl to massage user manual

The "fix-texi.perl" script is used to fix up the output of
`docbook2x-texi`:

  - It changes the filename to be "git.info".

  - It changes the directory category and entry.

The script is written in Perl, but it can be rather trivially converted
to a shell script. Do so to remove the dependency on Perl for building
the user manual.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>