Limit tab completion to text before the cursor

`substr` takes a count, so the text after the cursor joined the prefix.

test/src/test_input_path_input.h

@@ -0,0 +1,24 @@
+#include "test/helpers/test_fixture.h"
+
+#include <string>
+
+class TestInputPathInput : public test_fixture {
+  CPPUNIT_TEST_SUITE(TestInputPathInput);
+
+  CPPUNIT_TEST(test_complete_at_end);
+  CPPUNIT_TEST(test_complete_mid_line);
+  CPPUNIT_TEST(test_cursor_on_separator);
+
+  CPPUNIT_TEST_SUITE_END();
+
+public:
+  void setUp();
+  void tearDown();
+
+  void test_complete_at_end();
+  void test_complete_mid_line();
+  void test_cursor_on_separator();
+
+private:
+  std::string m_temp_dir;
+};