csplit: rewrite with one function per pattern type

[jfinkels]

This pull request fixes several bugs in csplit by rewriting the code to have one function per pattern type. The functions are

• up_to_line_repeat for patterns like 123 {45}
• up_to_line_always for 123 {*}
• up_to_match_pos_offset_repeat for /regex/+123 {45}
• up_to_match_pos_offset_always for /regex/+123 {*}
• up_to_match_neg_offset_repeat for /regex/-123 {45}
• up_to_match_neg_offset_always for /regex/-123 {*}
• skip_to_match_pos_offset_repeat for %regex%+123 {45}
• skip_to_match_pos_offset_always for %regex%+123 {*}
• skip_to_match_neg_offset_repeat for %regex%-123 {45}
• skip_to_match_neg_offset_always for %regex%-123 {*}

The command-line options, global state, and per-chunk state live in the Splitter struct (which replaces the SplitWriter and InputSplitter structs). There are a ton of ugly corner cases where different types of patterns interact. I've tried to call those out with comments.

I added some tests that were already passing, but were missing test coverage, including

• same regex pattern twice in a row, as in /foo/ /foo/
• line number out of range on empty input, as in : | csplit - 1,
• skip to match with negative offset repeating indefinitely, as in csplit %foo%-5 {*}.

Some bugs fixed include:

• missing a regex match on the final line of a line number pattern, as in seq 20 | csplit - 10 /10/,
• missing output 0\n when input is a directory,
• #7286, a missing empty file with --suppress-matched.

Some bugs remain:

• The GNU test file tests/csplit/csplit-suppress-matched remains failing. I'm hoping that the change in this pull request will make it easier to reason about csplit and fix the remaining failures.
• Probably related to the previous item, there is some weird behavior of GNU csplit that I could not understand. This was already partially covered by a test named repeat_everything. I've extracted just the relevant part of that into its own test named test_repeat_line_after_match_suppress_matched that has TODO comments for the behavior that differs from GNU csplit.

Fixes #7286

[github-actions[bot]]

GNU testsuite comparison:

GNU test failed: tests/misc/tee. tests/misc/tee is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions[bot]]

GNU testsuite comparison:

GNU test failed: tests/misc/tee. tests/misc/tee is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[sylvestre]

It is still in draft mode :)

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[jfinkels]

Sorry, I was hoping to find a way to make this easier to review, but I couldn't. I will come back to this and rebase it

[sylvestre]

it has been draft for a long time, please reopen when ready