Add a getter for a list of RefSpecs to Config

Add a getter for a list of RefSpecs to Config

Reading RefSpecs from a Config can be seen as another typed value
conversion, so add a getter to Config and to TypedConfigGetter. Use
it in RemoteConfig.

Doing this allows clients of the JGit library to customize the
handling of invalid RefSpecs in git config files by installing a
custom TypedConfigGetter.

Bug: 517314
Change-Id: I0ebc0f073fabc85c2a693b43f5ba5962d8a795ff
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>