Use XDG data directory for fish import (#851)

* Use XDG data directory for fish import

On MacOS, atuin was looking for fish history under "$HOME/Library/Application Support".

Now atuin honors XDG_DATA_HOME, if set, and otherwise uses "$HOME/.local/share".

* cargo fmt

---------

Co-authored-by: Charles Gould <charles@gould.dev>

atuin-client/src/import/fish.rs

@@ -19,7 +19,10 @@ pub struct Fish {
 /// see https://fishshell.com/docs/current/interactive.html#searchable-command-history
 fn default_histpath() -> Result<PathBuf> {
     let base = BaseDirs::new().ok_or_else(|| eyre!("could not determine data directory"))?;
-    let data = base.data_local_dir();
+    let data = std::env::var("XDG_DATA_HOME").map_or_else(
+        |_| base.home_dir().join(".local").join("share"),
+        PathBuf::from,
+    );
 
     // fish supports multiple history sessions
     // If `fish_history` var is missing, or set to `default`, use `fish` as the session