Do not expose HISTDB_FILE path via `ps`

[mafredri]

Sorry for spamming the PRs 😅. Anyway, this is a "nice to have", not an issue per-se, so I'll understand if it's of no interest to you to merge.

I prefer not to expose the full path of $HISTDB_FILE to process monitors. The use case for this is shared systems mostly where I might not want to expose the full path to the file. With this PR, the following:

sqlite3 -batch /Users/maf/.histdb/zsh-history.db

becomes:

sqlite3 -batch zsh-history.db

It's just neater (IMO) and doesn't expose custom paths if I were to load it from another place.

[larkery]

It does look neater. I can't think of a reason why it would be a problem, but I will give it a little time to percolate in my brain just in case.

Thanks