add.c: extract new die_if_path_beyond_symlink() for reuse

This will be reused by a new git check-ignore command.

Also document validate_pathspec().

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pathspec.c

@@ -87,3 +87,15 @@ const char *check_path_for_gitlink(const char *path)
 	}
 	return path;
 }
+
+/*
+ * Dies if the given path refers to a file inside a symlinked
+ * directory in the index.
+ */
+void die_if_path_beyond_symlink(const char *path, const char *prefix)
+{
+	if (has_symlink_leading_path(path, strlen(path))) {
+		int len = prefix ? strlen(prefix) : 0;
+		die(_("'%s' is beyond a symbolic link"), path + len);
+	}
+}