Print "git diff" output when the checksums tests has failed
In some cases this output is useful to identify the root of the problem. See https://github.com/postmarketOS/pmbootstrap/pull/303#issuecomment-319017197
In some cases this output is useful to identify the root of the problem. See https://github.com/postmarketOS/pmbootstrap/pull/303#issuecomment-319017197