GCMemcard: Implement ExportFile() to get a file on a card with a single method call.

Source/Core/Core/HW/GCMemcard/GCMemcard.cpp

@@ -909,6 +909,22 @@ GCMemcardImportFileRetVal GCMemcard::ImportFile(const DEntry& direntry,
   return GCMemcardImportFileRetVal::SUCCESS;
 }
 
+std::optional<Savefile> GCMemcard::ExportFile(u8 index) const
+{
+  if (!m_valid || index >= DIRLEN)
+    return std::nullopt;
+
+  Savefile savefile;
+  savefile.dir_entry = GetActiveDirectory().m_dir_entries[index];
+  if (savefile.dir_entry.m_gamecode == DEntry::UNINITIALIZED_GAMECODE)
+    return std::nullopt;
+
+  if (GetSaveData(index, savefile.blocks) != GCMemcardGetSaveDataRetVal::SUCCESS)
+    return std::nullopt;
+
+  return savefile;
+}
+
 GCMemcardRemoveFileRetVal GCMemcard::RemoveFile(u8 index)  // index in the directory array
 {
   if (!m_valid)

Source/Core/Core/HW/GCMemcard/GCMemcard.h

@@ -505,6 +505,9 @@ public:
   // adds the file to the directory and copies its contents
   GCMemcardImportFileRetVal ImportFile(const DEntry& direntry, std::vector<GCMBlock>& saveBlocks);
 
+  // Fetches the savefile at the given directory index, if any.
+  std::optional<Savefile> ExportFile(u8 index) const;
+
   // delete a file from the directory
   GCMemcardRemoveFileRetVal RemoveFile(u8 index);