Escape backslashes in fish completion descriptions

[greymoth-jp]

The fish completions generator escapes single quotes in each option and command description, but leaves backslashes untouched:

def sanitize(s: str) -> str:
    return self._io.output.formatter.remove_format(s).replace("'", "\'")

The result is emitted inside a single-quoted fish argument, -d '<description>'. Single quotes in fish are not fully literal: per the language docs, "the only meaningful escape sequences in single quotes are \', which escapes a single quote and \, which escapes the backslash symbol." So a backslash in a description is mishandled:

• a doubled backslash collapses to a single one, and
• a trailing backslash escapes the closing quote, which ends the string early and breaks the rest of the generated complete line.

The zsh path already accounts for this. _zsh_describe escapes backslashes (they are part of the character class it passes through re.sub), so the fish path is the odd one out.

The fix escapes backslashes before single quotes, matching the order zsh already uses:

return (
    self._io.output.formatter.remove_format(s)
    .replace("\\", "\\\\")
    .replace("'", "\'")
)

I added a regression test that renders a fish completion for a command whose description contains a backslash and a single quote, and checks that both are escaped. Reverting the one-line change makes it fail. The full test suite passes and ruff check/ruff format are clean.