diff_print: do not call abort(3P)

Calling abort(3P) in a library is rather rude and shouldn't happen, as
we effectively prohibit any corrective actions made by the application
linking to it. We thus shouldn't call it at all, but instead use our new
`GIT_ASSERT` macros.

Remove the call to abort(3P) in case a diff delta has an unexpected type
to fix this.

Author: pks-t

src/diff_print.c

@@ -351,12 +351,11 @@ static int diff_delta_format_similarity_header(
 		goto done;
 	}
 
+	GIT_ASSERT(delta->status == GIT_DELTA_RENAMED || delta->status == GIT_DELTA_COPIED);
 	if (delta->status == GIT_DELTA_RENAMED)
 		type = "rename";
-	else if (delta->status == GIT_DELTA_COPIED)
-		type = "copy";
 	else
-		abort();
+		type = "copy";
 
 	if ((error = git_buf_puts(&old_path, delta->old_file.path)) < 0 ||
 	    (error = git_buf_puts(&new_path, delta->new_file.path)) < 0 ||