refactor(git): unify around the Repo abstraction

after/ftplugin/git.lua

@@ -1,17 +0,0 @@
--- The built-in `git` filetype is reused for our `:G show` / `:G cat-file -p`
--- output buffers and for commits opened from the log window. We set
--- `vim.b.git_worktree` before assigning the filetype on those buffers; the
--- guard below keeps the <CR> dispatcher off any unrelated git buffer (a
--- real `.git/HEAD` file, etc.) so the default normal-mode <CR> still works.
-if not vim.b.git_worktree then
-    return
-end
-
-local cr = vim.api.nvim_replace_termcodes("<CR>", true, false, true)
-
-vim.keymap.set("n", "<CR>", function()
-    if not require("git.object").open_under_cursor() then
-        -- "n" mode = no remap, so this doesn't recurse into our mapping.
-        vim.api.nvim_feedkeys(cr, "n", false)
-    end
-end, { buffer = 0, silent = true, desc = "Open file at commit" })