The ZSH completion script of `tig` basically imports the bash completion
script and uses it as completion approach. Unfortunately the script
takes several assumptions about the directory structure using
`$funcsourcetrace`[1] that don't apply on NixOS.
The easiest workaround is t opatch the completion script and import the
bash completion script from `$out`.
[1] http://zsh.sourceforge.net/Doc/Release/Zsh-Modules.html#index-funcsourcetrace
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