fsck: do not reuse child_process structs

fsck: do not reuse child_process structs

The run-command API makes no promises about what is left in a struct
child_process after a command finishes, and it's not safe to simply
reuse it again for a similar command. In particular:

 - if you use child->args or child->env_array, they are cleared after
   finish_command()

 - likewise, start_command() may point child->argv at child->args->argv;
   reusing that would lead to accessing freed memory

 - the in/out/err may hold pipe descriptors from the previous run

These two calls are _probably_ OK because they do not use any of those
features. But it's only by chance, and may break in the future; let's
reinitialize our struct for each program we run.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>