Fix bash completion errors

[micl2e2]

When being used in conjunction with nim's completion scripts in such style:

source /path/to/nim.bash-completion
source /path/to/nimble.bash-completion

, and there would be an error while completing nim:

<TAB>
bash: ilastlast: substring expression < 0

nim's completion fails because both share the same helper functions: __is_short_or_long, __ask_for_subcmd_or_subopts, __ask_for_subcmd, __ask_for_subcmd_opts, and the impl of nimble's ones are a little bit more outdated than nim's ones, and multiple sources will overide the previous definition. This pr simply reuses those functions in nim, nimgrep, etc.