rust-hgignore: add a scripting command to print the hgignore regexp Add a command `script::hgignore --print-re` to print the hgignore regexp. One complication is that the `rootfilesin`-only matcher doesn't use a regular expression, and the existing converts it to something that's not a regular expression. We add code to handle that case. Since this command is now sufficient to generate a tidy-looking regexp for scripting, this frees up the "debug" command to report the internal regexp used by the regex engine, so we make that change too.