This commit refactors the previous `Commit` interface into a
standardised `Commit` struct. This object contains sufficient
information for referencing, observating and (PGP) verification.
- `libgit2` commit checkout does now return `HEAD/<SHA1>` as
the branch is not taken into account.
- `git2go` objects are now properly `Free`d everywhere
- `Verify` logic is tested.
Signed-off-by: Hidde Beydals <hello@hidde.co>