Better output for zero or one repositories

If there is no activity, display a suitable message.
If only one repository has activity, don't add a heading for it.

README.md

@@ -3,6 +3,16 @@ The idea is that you keep a journal (e.g. `log.md`) and instead of manually copy
 into it, you just run a command in your text editor and it pastes in everything you did since the last
 paste. You can then edit this text as required.
 
+For example, after creating this repository and making a PR against it, I type `\a` in Vim and it inserts:
+
+```
+Created repository [talex5/get-activity](https://github.com/talex5/get-activity).
+
+Better output for zero or one repositories [#1](https://github.com/talex5/get-activity/pull/1).  
+If there is no activity, display a suitable message. If only one repository
+has activity, don't add a heading for it.
+```
+
 ## Activity sources
 
 At the moment, GitHub activity is the only source it queries.

contributions.ml

@@ -174,13 +174,17 @@ let pp_body f = function
   | body -> Fmt.pf f "  @,@[<hov>%a@]" Fmt.words body
 
 let pp_item f t =
-  Fmt.pf f "@[<v>%a%a@]" pp_title t pp_body t.body
+  Fmt.pf f "@[<v>%a.%a@]" pp_title t pp_body t.body
 
-let pp_repo f (name, items) =
-  Fmt.pf f "### %s@,@,%a" name Fmt.(list ~sep:(cut ++ cut) pp_item) items
-
-let items_by_repo ~from json =
-  let t = of_json ~from:(to_8601 from) json in
-  Repo_map.bindings t
+let pp_items = Fmt.(list ~sep:(cut ++ cut) pp_item)
 
-let pp ~from = Fmt.(using (items_by_repo ~from) (list ~sep:(cut ++ cut) pp_repo))
+let pp_repo f (name, items) =
+  Fmt.pf f "### %s@,@,%a" name pp_items items
+
+let pp ~from f json =
+  let from = to_8601 from in
+  let by_repo = Repo_map.bindings @@ of_json ~from json in
+  match by_repo with
+  | [] -> Fmt.pf f "(no activity found since %s)" from
+  | [(_, items)] -> pp_items f items
+  | repos -> Fmt.(list ~sep:(cut ++ cut)) pp_repo f repos