giteapc: disallow install with no arguments and no -u (#7)

Fixes: #7

giteapc/repo.py

@@ -329,7 +329,9 @@ def install(*args, use_https=False, use_ssh=False, update=False, yes=False,
     dry_run=False):
 
     # Update all repositories
-    if args == () and update == True:
+    if args == () and not update:
+        return fatal(f"repo install: need one argument (unless -u is given)")
+    if args == () and update:
         args = [ r.fullname for r in LocalRepo.all() ]
 
     # Fetch every repository and determine its dependencies to form a basic