hooks are ignored if there are not marked as executable

[Damien <damien@xxxxxx>]

Hi,

If you do `echo my_script > .git/hooks/pre-commit` and then `git commit`,
The hook is just gonna be ignored.
But if you do `chmod +x .git/hooks/pre-commit`, then it's executed.

I think ignoring a hook is misleading and not newbie friendly, an error
message to signal an incorrectly configured hook could be better.
At least as a warning to be backward-compatible.

Thanks,
Damien