Avoid running rbenv rehash multiple times during bundle install

This is an attempt to work around the fact that Rubygems post_install
hooks may happen multiple times per single `bundle install` and ideally
we want `rbenv rehash` to run only once if new gems have been installed.
However, due to Bundler parallelism using `fork` on platforms that
support it, it's impossible for the child processes to communicate with
the master process to signal it to run `rbenv rehash` in the end.

This hooks into Bundler `install` command and runs `rbenv rehash` after
all gems have finished installing, but only if the install location was
system gems location and not a custom path (such as per-project
`vendor/bundle`).

This is limited because we can't tell whether any gems have been
installed at all, let alone do those gems have executables. However it's
better than having multiple `rbenv rehash` being run in parallel and
outputting confusing error messages as a result.

rbenv.d/exec/gem-rehash/rubygems_plugin.rb

@@ -6,13 +6,32 @@ hook = lambda do |installer|
       `rbenv rehash`
     end
   rescue
-    warn "rbenv: error in gem-rehash (#{$!})"
+    warn "rbenv: error in gem-rehash (#{$!.class.name}: #{$!.message})"
   end
 end
 
-begin
-  Gem.post_install(&hook)
-  Gem.post_uninstall(&hook)
-rescue
-  warn "rbenv: error installing gem-rehash hooks (#{$!})"
+if defined?(Bundler::Installer) && Bundler::Installer.respond_to?(:install)
+  Bundler::Installer.class_eval do
+    class << self
+      alias install_without_rbenv_rehash install
+      def install(root, definition, options = {})
+        result = install_without_rbenv_rehash(root, definition, options)
+        begin
+          if result && Gem.default_path.include?(Bundler.bundle_path.to_s)
+            `rbenv rehash`
+          end
+        rescue
+          warn "rbenv: error in Bundler post-install hook (#{$!.class.name}: #{$!.message})"
+        end
+        result
+      end
+    end
+  end
+else
+  begin
+    Gem.post_install(&hook)
+    Gem.post_uninstall(&hook)
+  rescue
+    warn "rbenv: error installing gem-rehash hooks (#{$!.class.name}: #{$!.message})"
+  end
 end