auto-mirror: several fixes to make the mirror work and better

1. make it not fail when the previous sha information failed to be fetched. This is mainly for the first time when no previous sha exist.
2. fixed the issues on tuxtrigger config files, where squad_group is not used anymore, but still needs to be specified to avoid YamlValidator error
3. fixes "No space left on device" error, and also improved clone time when mirror to multiple repositories, and added the missed android14-5.15 branch
4. add some lines to print the timestamp and storage capacity information for future failures debug

Signed-off-by: Yongqin Liu yongqin.liu@linaro.org