set the executable bit on pre-commit.sh

`x.py setup` hardlinks this file into .git/hooks. Prior to this commit,
that led to the following warning emitted by `git commit`:

    hint: The '.git/hooks/pre-commit' hook was ignored because it's not set as executable.

Making the checked-in script executable fixes this issue, as the
hardlinked copy uses the same flags.

It looks like the file was originally executable, but that bit was
unset in commit b908905 of
#85305. It's possible that was
unintentional.