Fix parsing of git output with unusual characters

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.