check-executables-have-shebangs needs to check the actual file mode
known to Git and avoid relying only on the filesystem permissions,
because some filesystems force the executable bit on every file (e.g.
fat32 on Linux).
On Windows, all files are "executable".
Therefore, to know if a file is supposed to be executed,
we check how its attributes were recorded by git:
we run a `git ls-files` command in a subprocess.
By default, this command outputs information
on multiple lines (file and their data separated by newlines).
When a file contains an unusual character,
the character is escaped with an integer sequence
(such as `\303\261`), and git wraps the whole filename
in double-quotes because of the backslashes.
It breaks the current code because we try to open
the filename containing the double-quotes:
it doesn't exist, of course.
Instead of trying to fix this special case by removing
the double-quotes, and breaking other cases
(a double-quote is a valid filename character on Linux),
we tell git to separate each item with the null character `\0`
instead of a new line `\n`, with the option `-z`.
With this option, git doesn't escape unusual characters
with integer sequence, so the output is fixed, and we
parse it by splitting on `\0` instead of `\n`.
Fixes#508.