rbenv init: modify shell config files instead of printing instructions (#1568)

When running `rbenv init`, typically during rbenv setup, users expected
their shell environment to be modified permanently. Instead, what the
command would do is print the instructions to the user and expect them
to edit their shell initialization files accordingly. This proved to
be unintuitive.

Now, running `rbenv init <shells>...` will modify the shell initialization
files of the following shells:

- bash: `~/.bash_profile` or `~/.bashrc` if the latter exists but the former does not
- zsh: `~/.zprofile` or `~/.zshrc` if the latter exists and mentions "rbenv"
- fish: `~/.config/fish/config.fish`

If no shells were specified on the command line, rbenv will try to detect
the current shell.

It should be safe to run `rbenv init` multiple times, as the command will
avoid modifying any shell startup file that already mentions "rbenv init".

test/init.bats

@@ -51,18 +51,67 @@ OUT
   [ -z "$line" ] || flunk "did not expect line: $line"
 }
 
-@test "posix shell instructions" {
+@test "set up bash" {
+  assert [ ! -e ~/.bash_profile ]
   run rbenv-init bash
-  assert [ "$status" -eq 1 ]
+  assert_success "writing ~/.bash_profile: now configured for rbenv."
+  run cat ~/.bash_profile
+  # shellcheck disable=SC2016
   assert_line 'eval "$(rbenv init - bash)"'
 }
 
-@test "fish instructions" {
+@test "set up bash (bashrc)" {
+  mkdir -p "$HOME"
+  touch ~/.bashrc
+  assert [ ! -e ~/.bash_profile ]
+  run rbenv-init bash
+  assert_success "writing ~/.bashrc: now configured for rbenv."
+  run cat ~/.bashrc
+  # shellcheck disable=SC2016
+  assert_line 'eval "$(rbenv init - bash)"'
+}
+
+@test "set up zsh" {
+  unset ZDOTDIR
+  assert [ ! -e ~/.zprofile ]
+  run rbenv-init zsh
+  assert_success "writing ~/.zprofile: now configured for rbenv."
+  run cat ~/.zprofile
+  # shellcheck disable=SC2016
+  assert_line 'eval "$(rbenv init - zsh)"'
+}
+
+@test "set up zsh (zshrc)" {
+  unset ZDOTDIR
+  mkdir -p "$HOME"
+  cat > ~/.zshrc <<<"# rbenv"
+  run rbenv-init zsh
+  assert_success "writing ~/.zshrc: now configured for rbenv."
+  run cat ~/.zshrc
+  # shellcheck disable=SC2016
+  assert_line 'eval "$(rbenv init - zsh)"'
+}
+
+@test "set up fish" {
+  unset XDG_CONFIG_HOME
   run rbenv-init fish
-  assert [ "$status" -eq 1 ]
+  assert_success "writing ~/.config/fish/config.fish: now configured for rbenv."
+  run cat ~/.config/fish/config.fish
   assert_line 'status --is-interactive; and rbenv init - fish | source'
 }
 
+@test "set up multiple shells at once" {
+  unset ZDOTDIR
+  unset XDG_CONFIG_HOME
+  run rbenv-init bash zsh fish
+  assert_success
+  assert_output <<OUT
+writing ~/.bash_profile: now configured for rbenv.
+writing ~/.zprofile: now configured for rbenv.
+writing ~/.config/fish/config.fish: now configured for rbenv.
+OUT
+}
+
 @test "option to skip rehash" {
   run rbenv-init - --no-rehash
   assert_success