Use start_command() to run content filters instead of explicit fork/exec.

Use start_command() to run content filters instead of explicit fork/exec.

The previous code already used finish_command() to wait for the process
to terminate, but did not use start_command() to run it.

Signed-off-by: Johannes Sixt <redacted>
Signed-off-by: Shawn O. Pearce <redacted>