Improve rbenv init instructions for fish

Fish user config file `~/.config/fish/config.fish` loads for every
instance of fish shell, not just interactive ones. Since it's
unnecessary and dangerous to eval `rbenv init -` output in
non-interactive shells, wrap the invocation in a conditional that checks
if the current shell is interactive.

Fixes #501

libexec/rbenv-init

@@ -77,7 +77,7 @@ if [ -z "$print" ]; then
     echo
     case "$shell" in
     fish )
-      echo '. (rbenv init -|psub)'
+      echo 'status --is-interactive; and . (rbenv init -|psub)'
       ;;
     * )
       echo 'eval "$(rbenv init -)"'

test/init.bats

@@ -38,6 +38,12 @@ load test_helper
   assert_line ". '${root}/libexec/../completions/rbenv.fish'"
 }
 
+@test "fish instructions" {
+  run rbenv-init fish
+  assert [ "$status" -eq 1 ]
+  assert_line 'status --is-interactive; and . (rbenv init -|psub)'
+}
+
 @test "option to skip rehash" {
   run rbenv-init - --no-rehash
   assert_success