Show the output of Git pull when updating plugins

[FranklinYu]

This tells users whether a plugin changed during the update. When update succeed, it shows something like

Updating all plugins!

  "tmux-resurrect" update success
    | Already latest.
  "foo-bar" update success
    | Already latest.

When it fail, it shows something like

Updating all plugins!

  "tmux-resurrect" update fail
    | remote: Support for password authentication was removed on August 13, 2021.
    | Fatal: 'https://github.com/tmux-plugins/tmux-resurrectt/' authentication fail
  "foo-bar" update success
    | Already latest.

Difference from #84:

1. Make the variable local.
2. Put the status after the main message, so that it looks like a tree (after indentation).

Decision that requires last call from maintainer: whether we want to always show it, or hide it behind some command-line flag (or environment variable, or tmux configuration variable).

Fixes #76.

[bruno-]

Thanks