Improve interactiveness checks (#3449)

* [bash] return instead of not executing an if-block, when non-interactive

This should keep the code more readable, be less error prone (accidentally doing
something outside the if-block and aligns the code with what’s already done for
zsh.

`0` is returned, because it shall not be considered an error when the script is
(accidentally) sourced from a non-interactive shell.

If executed as a script (rather than sourced), the results are not specified by
POSIX but depend on the shell, with bash giving an error in that case.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>

* [shell] exit immediately when called from non-interactive shell

The shell execution environment shouldn’t be modified at all, when called from a
non-interactive shell.

It shall be noted that the current check may become error prone for bash, namely
in case there should ever be a differentiation between `i` and `I` in the
special variable `-` and bash’s `nocasematch`-shell-option be used.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>

shell/completion.zsh

@@ -9,6 +9,9 @@
 # - $FZF_COMPLETION_TRIGGER (default: '**')
 # - $FZF_COMPLETION_OPTS    (default: empty)
 
+[[ -o interactive ]] || return 0
+
+
 # Both branches of the following `if` do the same thing -- define
 # __fzf_completion_options such that `eval $__fzf_completion_options` sets
 # all options to the same values they currently have. We'll do just that at
@@ -73,9 +76,6 @@ fi
 # `finally` in lesser languages. We use it to *always* restore user options.
 {
 
-# Bail out if not interactive shell.
-[[ -o interactive ]] || return 0
-
 # To use custom commands instead of find, override _fzf_compgen_{path,dir}
 if ! declare -f _fzf_compgen_path > /dev/null; then
   _fzf_compgen_path() {