Add a few reference utility functions
Allow for a default in reflog messages

We don't have a way to represent a NULL string, so if the user passes an
empty string, let's pass NULL down so we tell libgit2 to use the default.
Merge pull request #60 from libgit2/cmn/reflogs-default

Allow for a default in reflog messages
Merge pull request #59 from libgit2/cmn/refs

Add a few reference utility functions
fix bad merge (LastError -> MakeGitError)
wrap merge functions
merge options / merge tree options
fix bad git_buf handling
Remove unused consts
fix msg handling to treat empty str as nil
fix handling of msg to treat empty str as nil
move return outside of switch for go 1.0 / travis
switch from iota to explicit def
move finalizer above free
Merge branch 'master' into jezell/working-branch2
merge
Merge branch 'add-branch-lookup' into jezell/working-branch2
Merge branch 'remotes-wip' into jezell/working-branch2
merge updates