`git stash --include-untracked` touches .pack files

`git stash --include-untracked` causes the timestamp to be touched for 
some .pack files.  It's not always all .pack files, although I don't see 
a pattern regarding how some survive the operation with the timestamp 
intact.  For a given repo it seems consistent though on the .pack files 
impacted by the issue.

This breaks backup systems such as rsync, causing pointless churn.

The issue occurs with or without ".keep" files for the ".pack" files in 
question.

`git stash` without `--include-untracked` does not have this issue.

The issue occurs with no untracked files actually even in the repo for 
`--include-untracked` to take action on.