Blame: improve error messages for the , operation

The message is now:

   Commit f4657f0 has no ancestors modifying the seleted line

instead of  

   The selected commit has no parents

This addresses a portion of jonas#1315.

Apart from being a bit clearer, the user will be able
to learn what `,` is meant to do from it. Trying to
press `,` a few times is a natural thing to try to
do after reading the help while trying to figure
out how blame works in tig.

src/blame.c

@@ -355,12 +355,12 @@ blame_go_forward(struct view *view, struct blame *blame, bool parent)
 	const char *filename = parent ? commit->parent_filename : commit->filename;
 
 	if (!*id && parent) {
-		report("The selected commit has no parents");
+		report("Commit %.8s has no ancestors modifying the seleted line", commit->id);
 		return;
 	}
 
 	if (!strcmp(history_state->id, id) && !strcmp(history_state->filename, filename)) {
-		report("The selected commit is already displayed");
+		report("Commit %.8s is already beind displayed", commit->id);
 		return;
 	}