repo has been modified automatically after repo sync

Question

On macosx,I use below command to repo sync from google:

repo init -u https://android.googlesource.com/platform/manifest -b android-8.0.0_r4
repo sync frameworks/av

Then in frameworks/av, I found one file always modifed:

cd frameworks/av/
git status
Not currently on any branch.
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

    modified:   media/libstagefright/include/DataURISource.h

no changes added to commit (use "git add" and/or "git commit -a")

I can not remove that modified file at all, git reset --hard doesn't work.