fix(fish): clean up variables

* use `status dirname` builtin to save 1 dirname call
* use builtin test function instead of [ command
* use local scoping for INSTALL_DIR variable (and make it lowercase to
  indicate such change)
* remove $FORGIT as it's not needed anymore (we already set git-forgit
  alias)

Ref: https://fishshell.com/docs/current/cmds/test.html

Author: folliehiyuki

conf.d/forgit.plugin.fish

@@ -1,11 +1,9 @@
 # MIT (c) Chris Apple
 
-set INSTALL_DIR (dirname (dirname (status -f)))
-set -x FORGIT_INSTALL_DIR "$INSTALL_DIR/conf.d"
-set -x FORGIT "$FORGIT_INSTALL_DIR/bin/git-forgit"
-if [ ! -e "$FORGIT" ]
-    set -x FORGIT_INSTALL_DIR "$INSTALL_DIR/vendor_conf.d"
-    set -x FORGIT "$FORGIT_INSTALL_DIR/bin/git-forgit"
+set -l install_dir (dirname (status dirname))
+set -x FORGIT_INSTALL_DIR "$install_dir/conf.d"
+if not test -e "$FORGIT_INSTALL_DIR/bin/git-forgit"
+    set -x FORGIT_INSTALL_DIR "$install_dir/vendor_conf.d"
 end
 
 function forgit::warn