Document GIT_SSH environment variable alongside other variables
The GIT_SSH environment variable has survived for quite a while
without being documented, but has been mentioned on list and on
my day-job repositories can only be accessed via magic supplied
through the wonderous hack that is GIT_SSH.
Advertising it alongside other "low level magic" such as GIT_PAGER
and GIT_MERGE_VERBOSITY will certainly help others who need to
spread their own pixie dust to make things work.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>