partial-clone: define partial clone settings in config

Create get and set routines for "partial clone" config settings.
These will be used in a future commit by clone and fetch to
remember the promisor remote and the default filter-spec.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -28,6 +28,7 @@ int warn_on_object_refname_ambiguity = 1;
 int ref_paranoia = -1;
 int repository_format_precious_objects;
 char *repository_format_partial_clone;
+const char *core_partial_clone_filter_default;
 const char *git_commit_encoding;
 const char *git_log_output_encoding;
 const char *apply_default_whitespace;