Extending previous recording doesn't increment first block

[pmolfese]

SRT files have a block ordering code:

1
00:00:02,420 --> 00:00:04,500
- Hello, my name is Peter Molfese.

2
00:00:04,500 --> 00:00:05,500
I'm a scientist

When extending from a previous file, the block offset correctly changes the block ordering code for all but the first entry of each file.

./offset_caption --tail_from file1.srt --block_offset 1138 file2.srt file2_extended.srt

In this example, the output would be this:

1
00:00:09,420 --> 00:00:011,500
- Hello, my name is Peter Molfese.

1139
00:00:011,500 --> 00:00:12,500
I'm a scientist