hush: fix "set -x" output prefix overlapping for v="..`cmd`.." case

hush: fix "set -x" output prefix overlapping for v="..`cmd`.." case

Was printing initial "+" prefix for the assignment, that printing

	"+ cmd"

then printing the expanded " v=VAL" string.

Delay printing of "+" prefix for the assignment to after expansion.

function                                             old     new   delta
run_pipe                                            1883    1902     +19
builtin_eval                                         127     133      +6
expand_vars_to_list                                 1103    1106      +3
dump_cmd_in_x_mode                                   144     142      -2
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(add/remove: 0/0 grow/shrink: 3/1 up/down: 28/-2)              Total: 26 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>