run_command(): help callers distinguish errors

run_command(): help callers distinguish errors

run_command() returns a single integer specifying either an
error code or the exit status of the spawned program. The
only way to tell the difference is that the error codes are
outside of the allowed range of exit status values.

Rather than make each caller implement the test against a
magic limit, let's provide a macro.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>