__fish_complete_path: preserve equals signs in prefixes - #12908

Description

__fish_complete_path --bar= currently treats the equals sign as an option or assignment delimiter, then offers unrelated files with --bar= prepended. This escapes the equals sign only in the synthetic command line passed to complete, keeping it part of the path prefix. A real match such as --bar=baz still completes normally.

Fixes #12885.

Testing

• python3 tests/test_driver.py target/debug tests/checks/complete_path.fish
• target/debug/fish_indent --check share/functions/__fish_complete_path.fish tests/checks/complete_path.fish
• TERM=xterm-256color cargo xtask check passed formatting, Clippy, unit tests, doc tests, docs, and 204 of 207 integration checks. The remaining checks were blocked by local Sphinx deprecation warnings and a tmux process-group permission error; none exercise these files.

TODOs

• If addressing an issue, a commit message mentions Fixes issue #<issue-number>
• Changes to fish usage are reflected in user documentation/manpages. No user documentation changes are needed for this private completion helper.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst. Skipped for this completion-only change.