perf(git): fold revspec validation into the cat-file -p the loader needs anyway

lua/git/object.lua

@@ -135,16 +135,29 @@ local function attach_index_writer(buf, worktree, path)
     })
 end
 
+---Pre-fetched content keyed by bufnr. Set by `buf_for(_, _, content)`
+---and consumed by the next `read_uri` dispatch on that buffer. Lets
+---callers fold validation + content fetch + buffer load into one
+---`cat-file` call instead of preflighting separately.
+---@type table<integer, string>
+local pending_content = {}
+
 ---Return a buffer holding the content addressed by a git revspec. The
 ---URI is `git://<revspec>` and BufReadCmd routes through `M.read_uri`,
----which loads via `git cat-file -p`.
+---which loads via `git cat-file -p`. If `content` is given, primes a
+---cache so the BufReadCmd handler reuses it instead of running another
+---`cat-file -p`.
 ---@param worktree string
 ---@param revspec string  any revspec git understands (e.g. `HEAD:foo`, `:foo`, `:1:foo`, `<sha>`, `<sha>:foo`)
+---@param content string?
 ---@return integer
-function M.buf_for(worktree, revspec)
+function M.buf_for(worktree, revspec, content)
     local name = "git://" .. revspec
     local buf = vim.fn.bufadd(name)
     vim.b[buf].git_worktree = worktree
+    if content then
+        pending_content[buf] = content
+    end
     vim.fn.bufload(buf)
     return buf
 end
@@ -172,10 +185,14 @@ function M.read_uri(buf)
     vim.bo[buf].swapfile = false
     vim.bo[buf].bufhidden = "wipe"
 
-    local stdout = util.exec(
-        { "git", "cat-file", "-p", revspec },
-        { cwd = worktree }
-    )
+    local stdout = pending_content[buf]
+    pending_content[buf] = nil
+    if stdout == nil then
+        stdout = util.exec(
+            { "git", "cat-file", "-p", revspec },
+            { cwd = worktree }
+        )
+    end
     if stdout then
         vim.api.nvim_buf_set_lines(buf, 0, -1, false, util.split_lines(stdout))
     end