Problem: Repeatedly toggling `=` in fugitive left green gutter
extmarks on lines between sections. Two issues combined:
(1) carry_forward_highlighted returned an empty table (truthy in
Lua) when zero hunks matched, so pending_clear stayed false and
compressed extmarks from deleted diff lines were never cleared.
(2) The namespace clear in on_buf's pending_clear path was removed
in 2feb8a8, so even when pending_clear was true the extmarks
survived.
Solution: Return nil from carry_forward_highlighted when no hunks
were carried forward, so pending_clear is correctly set to true.
Restore nvim_buf_clear_namespace in on_buf's pending_clear block.
Add process_pending_clear test helper and spec coverage.
Barrett Ruth
330b3fdc28