syntax (completion-context): fix an infinite loop with three or more nested brace contexts

[dlyongemallo]

nest[3] stores nlen (the length to the parent nest), not the parent's absolute index. Following the same pattern as nest-pop, convert it via inest1-nest[3]. Without this, every _ble_syntax_nest[k] for k>=1 inside {{{...} has nlen=1, and the walk-outward loop becomes a fixed point at inest1=1.

Fixes #698.

[Copilot]

Pull request overview

Fixes an infinite loop in completion-context handling when walking outward through three or more nested brace-expansion contexts.

Changes:

• Converts nested brace parent links from stored relative nlen values to absolute indices while walking outward.
• Aligns brace completion-context traversal with existing nest-pop semantics.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[dlyongemallo]

The test failures seem unrelated to this PR.

[akinomyoga]

The CI issue turned out to be caused by the change of the OSTYPE special shell variable in the MSYS-2.0 environment (which is used by the Windows host in GitHub Actions). The Bash in the MSYS-2.0 environment has been setting OSTYPE=msys, but Bash in the current version of the MSYS-2.0 environment seems to set OSTYPE=cygwin. I'm not sure if this is an intentional change or a breakage of config.guess in Bash, but I added a workaround in commit e9bef3c.

[akinomyoga]

Thank you for the fix! I've merged the PR.