Bug Description

Commit 600fb13 ("Don't delete local untracked files when resetting worktree") introduced an O(N²) performance regression that affects every Clone operation.

During Clone, go-git internally performs a MergeReset (worktree.go:138). In v5.16.4, resetWorktree was called with a nil file list:

  // v5.16.4
  w.resetWorktree(t, opts.Files)  // opts.Files is nil during Clone

In v5.16.5, resetIndex now collects all touched files and passes them to resetWorktree:

  // v5.16.5
  removedFiles, _ = w.resetIndex(t, dirs, opts.Files)
  w.resetWorktree(t, removedFiles)

Inside resetWorktree, the inFiles function is called for every worktree change when the file list is non-nil:

  func inFiles(files []string, v string) bool {
      v = filepath.Clean(v)
      for _, s := range files {
          if filepath.Clean(s) == v {
              return true
          }
      }
      return false
  }

During a fresh Clone, both removedFiles and changes (from diffStagingWithWorktree) contain every file in the repository. This results in N×N calls to filepath.Clean, where every check returns true, the filtering has no practical effect during Clone, but the cost is paid in full.

For a repository with ~50,000 files, this produces ~2.5 billion filepath.Clean calls.
This affects every user of git.Clone / git.PlainClone - the MergeReset is hardcoded in the checkout path (worktree.go:138), not opt-in.

go-git Version

v5.16.5

Steps to Reproduce

No response

Additional Information

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