hush: improve set -x: make "+++" indent level increase in `cmd` and eval.

hush: improve set -x: make "+++" indent level increase in `cmd` and eval.

function                                             old     new   delta
dump_cmd_in_x_mode                                   126     144     +18
run_pipe                                            1873    1883     +10
builtin_eval                                         119     127      +8
expand_vars_to_list                                 1100    1103      +3
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(add/remove: 0/0 grow/shrink: 4/0 up/down: 39/0)               Total: 39 bytes

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