[progess_hook] information for already downloaded files.

added a new key 'alreadyexists' with value 'True' to dictionary passed
to progress_hook.

youtube_dl/downloader/common.py

@@ -338,6 +338,7 @@ class FileDownloader(object):
             self.report_file_already_downloaded(filename)
             self._hook_progress({
                 'filename': filename,
+                'alreadyexists':True,
                 'status': 'finished',
                 'total_bytes': os.path.getsize(encodeFilename(filename)),
             })

youtube_dl/downloader/http.py

@@ -106,6 +106,7 @@ class HttpFD(FileDownloader):
                             self.try_rename(tmpfilename, filename)
                             self._hook_progress({
                                 'filename': filename,
+                                'alreadyexists':True,
                                 'status': 'finished',
                                 'downloaded_bytes': resume_len,
                                 'total_bytes': resume_len,