src: Cap the maximum size of history registers

History registers (for prompt commands, pipe primitive commands etc) are
populated interactively by the users, and currently have no size limit.

This commits silently drops the oldest entries in the storage space to
allow at most 100 entries, to prevent long-running editing sessions from
hogging memory for data most likely never used.

src/register_manager.hh

@@ -85,6 +85,8 @@ class HistoryRegister : public StaticRegister
 public:
     void set(Context& context, ConstArrayView<String> values, bool restoring) override
     {
+        constexpr size_t size_limit = 100;
+
         if (restoring)
             return StaticRegister::set(context, values, true);
 
@@ -94,6 +96,10 @@ public:
                           m_content.end());
             m_content.push_back(entry);
         }
+
+        const size_t current_size = m_content.size();
+        if (current_size > size_limit)
+            m_content.erase(m_content.begin(), m_content.begin() + (current_size - size_limit));
     }
 
     const String& get_main(const Context&, size_t) override