csplit: incorrect split with --suppress-matched and positive offset

[jfinkels]

Environment: Ubuntu 20.04, uutils main branch (git commit 6a8b766), GNU coreutils v9.5.227-28b1.

Steps to reproduce:

printf "a\nb\nc\nd\ne\n" | ./target/debug/csplit - --suppress-matched /b/+1

What happens now: with uutils csplit the command outputs:

2
6

and the contents of the two files are

$ cat xx00
a
$ cat xx01
c
d
e

What I expected to happen: with GNU csplit, the command outputs

4
4

and the contents of the two files are

$ cat xx00
a
b
$ cat xx01
d
e

Notes: this is causing a test failure in GNU test file tests/csplit/csplit-suppress-matched.pl.

Also, the current erroneous behavior is enforced by a unit test in tests/by-util/test_csplit.rs named test_up_to_match_offset_option_suppress_matched. That test case will need to be updated as well.