git cache: disable cache usage when not running against the default branch

[radhermit]

To avoid non-linear history issues, git cache support should be disabled when it's determined the scan isn't being done in a linear fashion against the default branch for a known git cache.

[ionenwks]

This has made it harder to do branch comparison, it's giving me non-reproducible warnings (like nonexistentblocker) when not on default branch. Is there a way at most optionally make it use cache anyway? (do not wish to override those specific checks either).

Edit: well, guess this could also be a worry with people just dropping blockers thinking they were nonexistent, guess I'll just open a new bug.