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
rust-lang#85305. It's possible that was
unintentional.

Author: oconnor663