diff: allow unstuck arguments with --diff-algorithm
The argument to --diff-algorithm is mandatory, so there is no reason to
require the argument to be stuck to the option with '='. Change this
for consistency with other Git commands.
Note that this does not change the handling of diff-algorithm in
merge-recursive.c since the primary interface to that is via the -X
option to 'git merge' where the unstuck form does not make sense.
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>