RefDatabase: Introduce findRef synonym for getRef

RefDatabase: Introduce findRef synonym for getRef

Using findRef instead of getRef makes it clearer that the caller wants
to search for the ref in the search path, instead of looking for a ref
that exactly matches the input.

This change introduces the new findRef method and deprecates getRef.
It updates Repository#findRef to use the new method, ensuring some
test coverage.  Other callers will be updated in followup changes.

A nice side effect of introducing the new findRef method is that it is
final and based on firstExactRef, so implementers can focus on
implementing the latter efficiently and do not have to carefully write
custom path search code respecting SEARCH_PATH.

Change-Id: Id3bb944344a9743705fd1f20193ab679298fa51c
Signed-off-by: Jonathan Nieder <jrn@google.com>