revisit git block in metainfo

The reproducibility is not always guaranteed even if the reproducible flag is set to true. For instance, the commit hash may not exist in the history in case a branch has been rebased. We should revisit the current strategy in two regards:

1. the reproducible flag should only be set to true if the commit exists in the master branch of the repo (on top of the existing conditions);
2. more information on the commit should be stored so that one could still identify the commit despite a git rebase.