Update variable assignments to always use $(command "$arg")
in their RHS instead of "$(command "$arg")" as the latter
is harder to read. Make get_merge_tool_cmd() simpler by
avoiding "echo" and $(command) substitutions completely.
Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>