srt: generate unique name for backup file

[schdub]

If .bak file exist, then its contents are taken as the source file.

reproducing the error situation

# source file contents
$ cat 1.srt
1
00:00:01,111 --> 00:00:03,646
Line 1

2
00:00:07,569 --> 00:00:10,058
Line 2

# good
$ srt -i shift -10ms 1.srt && cat 1.srt
1
00:00:01,101 --> 00:00:03,636
Line 1

2
00:00:07,559 --> 00:00:10,048
Line 2

# fail
$ srt -i shift -10ms 1.srt && cat 1.srt
1
00:00:01,101 --> 00:00:03,636
Line 1

2
00:00:07,559 --> 00:00:10,048
Line 2

# fail
$ srt -i shift -20ms 1.srt && cat 1.srt
1
00:00:01,091 --> 00:00:03,626
Line 1

2
00:00:07,549 --> 00:00:10,038
Line 2

My solution is generate unique name for bak file.

[coveralls]

Coverage decreased (-0.4%) to 70.876% when pulling bfc452f on schdub:master into e23ca39 on byroot:master.

[coveralls]

Coverage decreased (-0.4%) to 70.876% when pulling bfc452f on schdub:master into e23ca39 on byroot:master.

[coveralls]

Coverage decreased (-0.4%) to 70.876% when pulling bfc452f on schdub:master into e23ca39 on byroot:master.