init-treemacs.el

;; list the packages you want
(setq package-list
      '(treemacs))

;; list the repositories containing them
(setq package-archives '(("gnu" . "https://elpa.gnu.org/packages/")
                         ("marmalade" . "https://marmalade-repo.org/packages/")
                         ("melpa" . "https://melpa.org/packages/")))

;; activate all the packages (in particular autoloads)
(package-initialize)

;; fetch the list of packages available 
(unless package-archive-contents
  (package-refresh-contents))

;; install the missing packages
(dolist (package package-list)
  (unless (package-installed-p package)
    (package-install package)))

;; treemacs
(global-set-key (kbd "M-0") 'treemacs-select-window)
(global-set-key (kbd "C-x t 1") 'treemacs-delete-other-windows)
(global-set-key (kbd "C-x t t") 'treemacs)
(global-set-key (kbd "C-x t B") 'treemacs-bookmark)
(global-set-key (kbd "C-x t C-t") 'treemacs-find-file)
(global-set-key (kbd "C-x t M-t") 'treemacs-find-tag)
(with-eval-after-load 'treemacs
  (setq treemacs-collapse-dirs              (if (executable-find "python") 3 0)
        treemacs-file-event-delay           5000
        treemacs-follow-after-init          t
        treemacs-follow-recenter-distance   0.1
        treemacs-goto-tag-strategy          'refetch-index
        treemacs-indentation                2
        treemacs-indentation-string         " "
        treemacs-is-never-other-window      nil
        treemacs-no-png-images              nil
        treemacs-project-follow-cleanup     nil
        treemacs-recenter-after-file-follow nil
        treemacs-recenter-after-tag-follow  nil
        treemacs-show-hidden-files          t
        treemacs-silent-filewatch           nil
        treemacs-silent-refresh             nil
        treemacs-sorting                    'alphabetic-desc
        treemacs-tag-follow-cleanup         t
        treemacs-tag-follow-delay           1.5
        treemacs-width                      35)

  (treemacs-follow-mode t)
  (treemacs-filewatch-mode t)
  (pcase (cons (not (null (executable-find "git")))
                (not (null (executable-find "python3"))))
    (`(t . t)
     (treemacs-git-mode 'extended))
    (`(t . _)
     (treemacs-git-mode 'simple))))