sync.c: download packages before starting commit

download_files sets the transaction state to STATE_DOWNLOADING.
Modifying the state after it has already been set to STATE_COMMITTING
created a brief window where SIGINT would fail to interrupt the process
and caused interrupted downloads to result in a 'transaction started'
message in the log with no matching transaction end.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>