Introduce exactRef to read a ref whose exact name is known

Introduce exactRef to read a ref whose exact name is known

Unlike getRef(name), the new exactRef method does not walk the search
path.  This should produce a less confusing result than getRef when the
exact ref name is known: it will not try to resolve refs/foo/bar to
refs/heads/refs/foo/bar even when refs/foo/bar does not exist.

It can be faster than both getRefs(ALL).get(name) and getRef(name)
because it only needs to examine a single ref.

A follow-up change will introduce a findRef synonym to getRef and
deprecate getRef to make the choice a caller is making more obvious
(exactRef or findRef, with the same semantics as getRefs(ALL).get and
getRefs(ALL).findRef).

Change-Id: If1bd09bcfc9919e7976a4d77f13184ea58dcda52
Signed-off-by: Jonathan Nieder <jrn@google.com>