This commit changes the `gogit` behavior for commit checkouts,
now allowing one to reference to just a commit while omitting any
branch reference. Doing this creates an Artifact with a
`HEAD/<commit>` revision.
If both a `branch` and `commit` are defined, the commit is expected
to exist within the branch. This results in a more efficient clone
of just the target branch, and also makes this change backwards
compatible.
Fixes#407Fixes#315
Signed-off-by: Hidde Beydals <hello@hidde.co>
b7376ce94c