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.

pre_commit_hooks/check_executables_have_shebangs.py

diff --git a/pre_commit_hooks/check_executables_have_shebangs.py b/pre_commit_hooks/check_executables_have_shebangs.py
index 1c50ea0..a02d2a9 100644
--- a/pre_commit_hooks/check_executables_have_shebangs.py
+++ b/pre_commit_hooks/check_executables_have_shebangs.py
@@ -12,6 +12,14 @@
 EXECUTABLE_VALUES = frozenset(('1', '3', '5', '7'))
 
 
+def zsplit(s: str) -> List[str]:
+    s = s.strip('\0')
+    if s:
+        return s.split('\0')
+    else:
+        return []
+
+
 def check_executables(paths: List[str]) -> int:
     if sys.platform == 'win32':  # pragma: win32 cover
         return _check_git_filemode(paths)
@@ -26,9 +34,9 @@
 
 
 def _check_git_filemode(paths: Sequence[str]) -> int:
-    outs = cmd_output('git', 'ls-files', '--stage', '--', *paths)
+    outs = cmd_output('git', 'ls-files', '-z', '--stage', '--', *paths)
     seen: Set[str] = set()
-    for out in outs.splitlines():
+    for out in zsplit(outs):
         metadata, path = out.split('\t')
         tagmode = metadata.split(' ', 1)[0]
 

tests/check_executables_have_shebangs_test.py

diff --git a/tests/check_executables_have_shebangs_test.py b/tests/check_executables_have_shebangs_test.py
index 5895a2a..7046081 100644
--- a/tests/check_executables_have_shebangs_test.py
+++ b/tests/check_executables_have_shebangs_test.py
@@ -73,6 +73,21 @@
         assert check_executables_have_shebangs._check_git_filemode(files) == 0
 
 
+def test_check_git_filemode_passing_unusual_characters(tmpdir):
+    with tmpdir.as_cwd():
+        cmd_output('git', 'init', '.')
+
+        f = tmpdir.join('mañana.txt')
+        f.write('#!/usr/bin/env bash')
+        f_path = str(f)
+        cmd_output('chmod', '+x', f_path)
+        cmd_output('git', 'add', f_path)
+        cmd_output('git', 'update-index', '--chmod=+x', f_path)
+
+        files = (f_path,)
+        assert check_executables_have_shebangs._check_git_filemode(files) == 0
+
+
 def test_check_git_filemode_failing(tmpdir):
     with tmpdir.as_cwd():
         cmd_output('git', 'init', '.')
@@ -87,6 +102,16 @@
         assert check_executables_have_shebangs._check_git_filemode(files) == 1
 
 
+@pytest.mark.parametrize('out', ('\0f1\0f2\0', '\0f1\0f2', 'f1\0f2\0'))
+def test_check_zsplits_correctly(out):
+    assert check_executables_have_shebangs.zsplit(out) == ['f1', 'f2']
+
+
+@pytest.mark.parametrize('out', ('\0\0', '\0', ''))
+def test_check_zsplit_returns_empty(out):
+    assert check_executables_have_shebangs.zsplit(out) == []
+
+
 @pytest.mark.parametrize(
     ('content', 'mode', 'expected'),
     (