Add correct file extension to screenshot.

src/git.cpp

@@ -149,8 +149,8 @@ void commit(const entry_type &entry)
 
     if (!entry.screenshot_filepath.empty())
     {
-        // TODO: Add real file extension.
-        fs::copy(entry.screenshot_filepath, basename + ".image");
+        const string extension{fs::path(entry.screenshot_filepath).extension()};
+        fs::copy(entry.screenshot_filepath, basename + extension);
     }
 
     // Add files.