feat(git): navigate any git object from cat-file headers

lua/git/object.lua

@@ -193,6 +193,63 @@ function M.open_commit(worktree, ref, opts)
     vim.bo[buf].filetype = "git"
 end
 
+---Open any git object in a buffer. Commits get the full
+---`M.open_commit` view (cat-file + diff-tree). Trees, blobs, and tags
+---dump `git cat-file -p` output as-is. The object type is detected via
+---`git cat-file -t`.
+---@param worktree string
+---@param ref string
+---@param opts ow.Git.OpenCommitOpts?
+function M.open_object(worktree, ref, opts)
+    local type_out = util.exec(
+        { "git", "cat-file", "-t", ref },
+        { cwd = worktree, silent = true }
+    )
+    local obj_type = type_out and vim.trim(type_out) or ""
+    if obj_type == "" then
+        util.warning("not a git object: %s", ref)
+        return
+    end
+
+    if obj_type == "commit" then
+        M.open_commit(worktree, ref, opts)
+        return
+    end
+
+    local split = opts and opts.split
+    local sha = repo.rev_parse(worktree, ref, true) or ref
+    local name = "git://" .. sha .. "//"
+    local existing = vim.fn.bufnr(name)
+    if existing ~= -1 and vim.api.nvim_buf_is_loaded(existing) then
+        if split == false then
+            vim.cmd.normal({ "m'", bang = true })
+            vim.api.nvim_set_current_buf(existing)
+        else
+            vim.api.nvim_open_win(existing, true, {
+                split = split or (vim.o.splitbelow and "below" or "above"),
+            })
+        end
+        return
+    end
+
+    local stdout = util.exec(
+        { "git", "cat-file", "-p", ref },
+        { cwd = worktree }
+    )
+    if not stdout then
+        return
+    end
+
+    local buf, _ = git.new_scratch({ name = name, split = split })
+    vim.b[buf].git_worktree = worktree
+    vim.b[buf].git_ref = sha
+    vim.bo[buf].modifiable = true
+    vim.api.nvim_buf_set_lines(buf, 0, -1, false, util.split_lines(stdout))
+    vim.bo[buf].modifiable = false
+    vim.bo[buf].modified = false
+    vim.bo[buf].filetype = "git"
+end
+
 ---@return boolean dispatched true if the cursor was on an actionable line
 function M.open_under_cursor()
     local ctx = context()
@@ -202,12 +259,16 @@ function M.open_under_cursor()
 
     local line = vim.api.nvim_get_current_line()
 
-    -- Cat-file header navigation. `parent <sha>` opens the referenced
-    -- commit. (`git show` doesn't emit a `parent` line, so this only
-    -- fires inside `M.open_commit` buffers.)
-    local parent_sha = line:match("^parent (%x+)$")
-    if parent_sha then
-        M.open_commit(ctx.worktree, parent_sha, { split = false })
+    -- Cat-file header navigation: `parent <sha>` (commit), `tree <sha>`
+    -- (commit / tag), `object <sha>` (tag's referent) all reference
+    -- another git object. Tree-entry lines `<mode> <type> <sha>\t<name>`
+    -- navigate to a child blob or subtree.
+    local sha = line:match("^parent (%x+)$")
+        or line:match("^tree (%x+)$")
+        or line:match("^object (%x+)$")
+        or line:match("^%d+ %w+ (%x+)\t.+$")
+    if sha then
+        M.open_object(ctx.worktree, sha, { split = false })
         return true
     end