perf: cache repo root and harden async paths (#100)

## Problem

`get_repo_root()` shells out to `git rev-parse` on every call, causing
4-6
redundant subprocesses per `gdiff_file()` invocation. Three other minor
issues: `highlight_vim_syntax()` leaks a scratch buffer if
`nvim_buf_call`
errors, `lib.ensure()` silently drops callbacks during download so hunks
highlighted mid-download permanently miss intra-line highlights, and the
debounce timer callback can operate on a deleted buffer.

## Solution

Cache `get_repo_root()` results by parent directory — repo roots don't
change within a session. Wrap `nvim_buf_call` and `nvim_buf_delete` in
pcall so the scratch buffer is always cleaned up. Replace the early
`callback(nil)` in `lib.ensure()` with a pending callback queue that
fires
once the download completes. Guard the debounce timer callback with
`nvim_buf_is_valid`.

lua/diffs/git.lua

@@ -1,13 +1,19 @@
 local M = {}
 
+local repo_root_cache = {}
+
 ---@param filepath string
 ---@return string?
 function M.get_repo_root(filepath)
   local dir = vim.fn.fnamemodify(filepath, ':h')
+  if repo_root_cache[dir] ~= nil then
+    return repo_root_cache[dir]
+  end
   local result = vim.fn.systemlist({ 'git', '-C', dir, 'rev-parse', '--show-toplevel' })
   if vim.v.shell_error ~= 0 then
     return nil
   end
+  repo_root_cache[dir] = result[1]
   return result[1]
 end
 

lua/diffs/highlight.lua

@@ -238,7 +238,7 @@ local function highlight_vim_syntax(bufnr, ns, hunk, code_lines, covered_lines,
 
   local spans = {}
 
-  vim.api.nvim_buf_call(scratch, function()
+  pcall(vim.api.nvim_buf_call, scratch, function()
     vim.cmd('setlocal syntax=' .. ft)
     vim.cmd('redraw')
 
@@ -256,7 +256,7 @@ local function highlight_vim_syntax(bufnr, ns, hunk, code_lines, covered_lines,
     spans = M.coalesce_syntax_spans(query_fn, code_lines)
   end)
 
-  vim.api.nvim_buf_delete(scratch, { force = true })
+  pcall(vim.api.nvim_buf_delete, scratch, { force = true })
 
   local hunk_line_count = #hunk.lines
   local extmark_count = 0

lua/diffs/init.lua

@@ -200,7 +200,9 @@ local function create_debounced_highlight(bufnr)
           timer = nil
           t:close()
         end
+        if vim.api.nvim_buf_is_valid(bufnr) then
           highlight_buffer(bufnr)
+        end
       end)
     )
   end

lua/diffs/lib.lua

@@ -8,6 +8,9 @@ local cached_handle = nil
 ---@type boolean
 local download_in_progress = false
 
+---@type fun(handle: table?)[]
+local pending_callbacks = {}
+
 ---@return string
 local function get_os()
   local os_name = jit.os:lower()
@@ -164,9 +167,10 @@ function M.ensure(callback)
     return
   end
 
+  table.insert(pending_callbacks, callback)
+
   if download_in_progress then
-    dbg('download already in progress')
+    dbg('download already in progress, queued callback')
-    callback(nil)
     return
   end
 
@@ -192,21 +196,25 @@ function M.ensure(callback)
   vim.system(cmd, {}, function(result)
     download_in_progress = false
     vim.schedule(function()
+      local handle = nil
       if result.code ~= 0 then
         vim.notify('[diffs] failed to download libvscode_diff', vim.log.levels.WARN)
         dbg('curl failed: %s', result.stderr or '')
-        callback(nil)
+      else
-        return
-      end
-
         local f = io.open(version_path(), 'w')
         if f then
           f:write(EXPECTED_VERSION)
           f:close()
         end
-
         vim.notify('[diffs] libvscode_diff downloaded', vim.log.levels.INFO)
-      callback(M.load())
+        handle = M.load()
+      end
+
+      local cbs = pending_callbacks
+      pending_callbacks = {}
+      for _, cb in ipairs(cbs) do
+        cb(handle)
+      end
     end)
   end)
 end