avformat/hlsenc: save the EXT-X-DISCONTINUITY from old list

avformat/hlsenc: save the EXT-X-DISCONTINUITY from old list

when use fix ticket 2nd problem.

command line test step:
rm -rf output*;./ffmpeg  -i ~/Movies/objectC/facebook.mp4 -an -c:v copy
-f hls -hls_time 4 -hls_list_size 5 -hls_flags +delete_segments
-hls_flags +append_list -hls_flags +omit_endlist -hls_flags
+discont_start -t 50 output.m3u8
./ffmpeg -i ~/Movies/objectC/facebook.mp4 -an -c:v libx264 -g 4 -f hls
-hls_time 4 -hls_list_size 5 -hls_flags +delete_segments -hls_flags
+append_list -hls_flags +omit_endlist -hls_flags +discont_start -t 5
output.m3u8
./ffmpeg -i ~/Movies/objectC/facebook.mp4 -an -c:v libx264 -g 4 -f hls
-hls_time 4 -hls_list_size 5 -hls_flags +delete_segments -hls_flags
+append_list -hls_flags +omit_endlist -hls_flags +discont_start -t 5
output.m3u8

result:
 #EXTM3U
 #EXT-X-VERSION:3
 #EXT-X-TARGETDURATION:5
 #EXT-X-MEDIA-SEQUENCE:6
 #EXTINF:4.120000,
 output6.ts
 #EXT-X-DISCONTINUITY
 #EXTINF:4.000000,
 output7.ts
 #EXTINF:0.960000,
 output8.ts
 #EXT-X-DISCONTINUITY
 #EXTINF:4.000000,
 output9.ts
 #EXTINF:0.960000,
 output10.ts

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>