allow shell command non-zero return code while using output

[QiBaobin]

Fix two issues found:

1. $ shell_keep_pipe doesn't work as the logic will stop while there is no-zero shell return code.
2. | pipe_to, ! shell_insert_output etc behavior different with expected, for instance, | then rg -o foo, we want all selections become 'foo' or empty instead of failing.

[jjoshm]

Just tried it because the $ was not working. fix working good thanks for providing!