Add command dispatcher

This simplifies the code in cgit.c and makes it easier to extend cgit with
new pages/commands.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cmd.h

@@ -0,0 +1,15 @@
+#ifndef CMD_H
+#define CMD_H
+
+typedef void (*cgit_cmd_fn)(struct cgit_context *ctx);
+
+struct cgit_cmd {
+	const char *name;
+	cgit_cmd_fn fn;
+	unsigned int want_repo:1,
+		want_layout:1;
+};
+
+extern struct cgit_cmd *cgit_get_cmd(struct cgit_context *ctx);
+
+#endif /* CMD_H */