Merge pull request #2085 from libgit2/rb/index-tree-blob-collision

Index tree-bob collision
Add reflog params to set-head calls
Ensure updating HEAD updates reflog
Enhance clarity
Ensure creating HEAD creates its reflog
Add passing reflog tests
Add reflog params to git_branch_create
Augment clone API with reflog parameters
Deleting a branch deletes its reflog
Add reflog parameters to git_branch_move
Ensure renaming a reference updates the reflog
Ensure moving a branch updates the reflog
Provide good default reflog messages in branch api
Document branch-creation reflog better
Enhance testing of signature parameters
Allow tests to run without user config