difftool: Add '-x' and as an alias for '--extcmd'

difftool: Add '-x' and as an alias for '--extcmd'

This adds '-x' as a shorthand for the '--extcmd' option.
Arguments to '--extcmd' can be specified separately, which
was not originally possible.

This also fixes the brief help text so that it mentions
both '-x' and '--extcmd'.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>