Adds a new public reference function `git_reference_lookup_oid`
that directly resolved a reference name to an OID without returning
the intermediate `git_reference` object (hence, no free needed).
Internally, this adds a `git_reference_lookup_resolved` function
that combines looking up and resolving a reference. This allows
us to be more efficient with memory reallocation.
The existing `git_reference_lookup` and `git_reference_resolve`
are reimplmented on top of the new utility and a few places in the
code are changed to use one of the two new functions.
This fixes an issue which was detected while using one of the libgit2 bindings [0]. The lack of the trailing forward slash led the name of references returned by git_reference_listall() to be prefixed with a forward slash.
[0]: https://github.com/libgit2/libgit2sharp/pull/108
