Fallback exactRef: Do not ignore symrefs to unborn branch

Fallback exactRef: Do not ignore symrefs to unborn branch

When asked to read a symref pointing to a branch-yet-to-be-born (such
as HEAD in a newly initialized repository), getRef and getRefs provide
different results.

	getRef: SymbolicRef[HEAD -> refs/heads/master=00000000]
	getRefs and getAdditionalRefs: nothing

exactRef should match the getRef behavior: it is meant to be a
simpler, faster version of getRef that lets you search for a ref
without resolving it using the search path without other semantic
changes.  But the fallback implementation of exactRef relies on getRefs
and produces null for this case.

Luckily the in-tree RefDatabase implementations override exactRef and
get the correct behavior.  But any out-of-tree storage backend that
doesn't inherit from DfsRefDatabase or RefDirectory would still return
null when it shouldn't.

Let the fallback implementation use getRef instead to avoid this.
This means that exactRef would waste some effort traversing the ref
search path when the named ref is not found --- but subclasses tend to
override exactRef for performance already, so in the default
implementation correctness is more important.

Bug: 478865
Change-Id: I60f04e3ce3bf4731640ffd2433d329e621330029