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/init.lua

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