CREDITS: redirect to Git log, remove current outdated content

All the removed names are already mentioned in the commit log, or are
listed as copyright holders in the contributed files.

Amongst the removed names, the following ones were not listed in the
commit log:

Mario Brito (Xan DPCM decoder)
Brian Foley (DSP utils optimizations)
Vladimir Gneushev (lavf/lavc)
Juan J. Sierralta (H.263)
Ewald Snel (qdm2, cinepak)
Lionel Ulmer (DSP utils optimizations)

CREDITS

-This file contains the names of some of the people who have contributed to
-FFmpeg. The names are sorted alphabetically by last name.  As this file is
-currently quite outdated and git serves as a much better tool for determining
-authorship, it remains here for historical reasons only.
+See the Git history of the project (git://source.ffmpeg.org/ffmpeg) to
+get the names of people who have contributed to FFmpeg.
 
-Dénes Balatoni
-Michel Bardiaux
-Fabrice Bellard
-Patrice Bensoussan
-Alex Beregszaszi
-BERO
-Thilo Borgmann
-Mario Brito
-Ronald Bultje
-Alex Converse
-Maarten Daniels
-Reimar Doeffinger
-Tim Ferguson
-Brian Foley
-Arpad Gereoffy
-Philip Gladstone
-Vladimir Gneushev
-Roine Gustafsson
-David Hammerton
-Wolfgang Hesseler
-Marc Hoffman
-Falk Hueffner
-Aurélien Jacobs
-Steven Johnson
-Zdenek Kabelac
-Robin Kay
-Todd Kirby
-Nick Kurshev
-Benjamin Larsson
-Loïc Le Loarer
-Daniel Maas
-Mike Melanson
-Loren Merritt
-Jeff Muizelaar
-Michael Niedermayer
-François Revol
-Peter Ross
-Måns Rullgård
-Stefano Sabatini
-Roman Shaposhnik
-Oded Shimon
-Dieter Shirley
-Konstantin Shishkov
-Juan J. Sierralta
-Ewald Snel
-Sascha Sommer
-Leon van Stuivenberg
-Roberto Togni
-Lionel Ulmer
-Reynaldo Verdejo
+To check the log, you can type the command "git log" in the FFmpeg
+source directory, or browse the online repository at
+http://source.ffmpeg.org.