When a document viewer that is unknown to the current version of git is
specified in the .git/config file, instead of erroring out the process
entirely, just issue a warning. It might be that the user usually is
using a newer git that supports it (and the configuration is written for
that version) but is temporarily using an older git that does not know the
viewer.
Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>