filter-branch documentation: non-zero exit status in command abort the filter

filter-branch documentation: non-zero exit status in command abort the filter

Since commit 8c1ce0f46b85d40f215084eed7313896300082df filter-branch fails
when a <command> has a non-zero exit status. This commit makes it clear
in the documentation and also fixes the parent-filter example, that was
incorrectly returning non-zero when the commit being tested wasn't the
one to be rewritten.

Signed-off-by: Caio Marcelo de Oliveira Filho <redacted>
Signed-off-by: Junio C Hamano <redacted>