Improve config handling for diff,submodules,attrs

This adds support for a bunch of core.* settings that affect
diff and status, plus fixes up some incorrect implementations
of those settings from before.  Also, this cleans up the
handling of config settings in the new submodules code and
in the old attrs/ignore code.

include/git2/submodule.h

@@ -22,7 +22,7 @@ GIT_BEGIN_DECL
 
 typedef enum {
 	GIT_SUBMODULE_UPDATE_CHECKOUT = 0,
-	GIT_SUBMOUDLE_UPDATE_REBASE = 1,
+	GIT_SUBMODULE_UPDATE_REBASE = 1,
 	GIT_SUBMODULE_UPDATE_MERGE = 2
 } git_submodule_update_t;
 
@@ -80,8 +80,6 @@ GIT_EXTERN(int) git_submodule_foreach(
 	int (*callback)(const char *name, void *payload),
 	void *payload);
 
-#define GIT_SUBMODULE_HEAD "[internal]HEAD"
-
 /**
  * Lookup submodule information by name or path.
  *