operations: ensure SrcFsType is set correctly when using --metadata-mapper

Before this change on files which have unknown length (like Google
Documents) the SrcFsType would be set to "memoryFs".

This change fixes the problem by getting the Copy function to pass the
src Fs into a variant of Rcat.

Fixes #7848

fs/object/object.go

@@ -195,6 +195,7 @@ type MemoryObject struct {
 	modTime time.Time
 	content []byte
 	meta    fs.Metadata
+	fs      fs.Fs
 }
 
 // NewMemoryObject returns an in memory Object with the modTime and content passed in
@@ -203,6 +204,7 @@ func NewMemoryObject(remote string, modTime time.Time, content []byte) *MemoryOb
 		remote:  remote,
 		modTime: modTime,
 		content: content,
+		fs:      MemoryFs,
 	}
 }
 
@@ -219,7 +221,16 @@ func (o *MemoryObject) Content() []byte {
 
 // Fs returns read only access to the Fs that this object is part of
 func (o *MemoryObject) Fs() fs.Info {
-	return MemoryFs
+	return o.fs
+}
+
+// SetFs sets the Fs that this memory object thinks it is part of
+// It will ignore nil f
+func (o *MemoryObject) SetFs(f fs.Fs) *MemoryObject {
+	if f != nil {
+		o.fs = f
+	}
+	return o
 }
 
 // Remote returns the remote path