Don't override PROMPT_DIRTRIM when path shortening is disabled

Closed Johann Dreo requested to merge github/fork/jaesivsm/develop into develop

Created by: jaesivsm

While testing for an advice I gave on issue #219 (closed), I realized that liquidprompt will override PROMPT_DIRTRIM settings even if we ask it not to modify the displayed path with LP_ENABLE_SHORTEN_PATH=0.

That's a minor issue but the fix is simple.

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