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/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