Fix a deadlock affecting fish_config

This fixes the following deadlock. The C++ functions path_get_config and
path_get_data lazily determine paths and then cache those in a C++ static
variable. The path determination requires inspecting the environment stack.
If these functions are first called while the environment stack is locked
(in this case, when fetching the $history variable) we can get a deadlock.

The fix is to call them eagerly during env_init. This can be removed once
the corresponding C++ functions are removed.

This issue caused fish_config to fail to report colors and themes.

Add a test.

tests/checks/invocation.fish

@@ -105,3 +105,6 @@ $fish --no-config -c 'echo notprinted | and true'
 # CHECKERR: echo notprinted | and true
 # CHECKERR:                   ^~^
 
+# Regression test for a hang.
+echo "set -L" | $fish > /dev/null
+