Set buffer filename if it has none and it is written with an explicit filename

src/core/Buffer.cc

@@ -487,3 +487,8 @@ EncodedString Buffer::get_string()
     m_gap_buffer->compact();
     return EncodedString(m_gap_buffer->address(0u), m_gap_buffer->size(), m_encoding);
 }
+
+void Buffer::set_filename(const std::string & filename)
+{
+    m_filename = std::make_shared<std::string>(filename);
+}

src/core/Buffer.h

@@ -183,6 +183,7 @@ public:
     void erase_code_point(const Buffer::Iterator & position);
     void erase_range(const Buffer::Range & range);
     std::shared_ptr<std::string> filename() const { return m_filename; }
+    void set_filename(const std::string & filename);
     Iterator begin() const { return Iterator(this); }
     std::shared_ptr<Iterator> beginp() const { return std::make_shared<Iterator>(this); }
     Iterator end() const { return *m_eof_iterator; }