It's somewhat common to try to write "/refs/tags/something". There is
no easy way to catch it during the main body of the function, as there
is no way to distinguish whether it's a leading slash or a double
slash somewhere in the middle.
Catch this at the beginning so we don't trigger the assert in
is_all_caps_and_underscore().
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.
