### Please describe the problem.
The recommended hook script does silently ignore files with non-ascii filenames.

### What steps will reproduce the problem?
see below.

### What version of git-annex are you using? On what operating system?
git-annex version: 6.20180112
on NixOS i686

### Please provide any additional information below.
[[!format txt """
[woffs@lapdoepp:/tmp] $ git init test
Leeres Git-Repository in /tmp/test/.git/ initialisiert

[woffs@lapdoepp:/tmp] $ cd test
[woffs@lapdoepp:/tmp/test] (master #) $ git annex init
init  ok
(recording state in git...)
[woffs@lapdoepp:/tmp/test] (master #) $ git config metadata.exiftool "CreateDate Model ImageSize FocusRange GPS FileType: NominalBitrate Title Artist Album Date: TrackNumber Year"l "CreateDate Model ImageSize FocusRange GPS FileType: NominalBitrate Title Artist Album Date: TrackNum
                                                                                                                                                        [woffs@lapdoepp:/tmp/test] (master #) $ git config annex.genmetadata true
                                                                                                                                                        [woffs@lapdoepp:/tmp/test] (master #) $ wget -O ./.git/hooks/pre-commit-annex http://git-annex.branchable.com/tips/automatically_adding_metadata/pre-commit-annex
--2018-02-26 10:28:42--  http://git-annex.branchable.com/tips/automatically_adding_metadata/pre-commit-annex
Auflösen des Hostnamens git-annex.branchable.com (git-annex.branchable.com)… 66.228.46.55, 2600:3c03::f03c:91ff:fedf:c0e5
Verbindungsaufbau zu git-annex.branchable.com (git-annex.branchable.com)|66.228.46.55|:80 … verbunden.
HTTP-Anforderung gesendet, auf Antwort wird gewartet … 200 OK
Länge: 3035 (3,0K)
Wird in »./.git/hooks/pre-commit-annex« gespeichert.

./.git/hooks/pre-commit-annex              100%[=====================================================================================>]   2,96K  --.-KB/s    in 0s      

2018-02-26 10:28:42 (75,8 MB/s) - »./.git/hooks/pre-commit-annex« gespeichert [3035/3035]

                                                                                                                                                        [woffs@lapdoepp:/tmp/test] (master #) $ chmod +x ./.git/hooks/pre-commit-annex
                                                                                                                                                        wget http://git-a[woffs@lapdoepp:/tmp/test] (master #) $ wget http://git-annex.branchable.com/logo_small.png
--2018-02-26 10:28:52--  http://git-annex.branchable.com/logo_small.png
Auflösen des Hostnamens git-annex.branchable.com (git-annex.branchable.com)… 66.228.46.55, 2600:3c03::f03c:91ff:fedf:c0e5
Verbindungsaufbau zu git-annex.branchable.com (git-annex.branchable.com)|66.228.46.55|:80 … verbunden.
HTTP-Anforderung gesendet, auf Antwort wird gewartet … 200 OK
Länge: 4749 (4,6K) [image/png]
Wird in »logo_small.png« gespeichert.

logo_small.png                             100%[=====================================================================================>]   4,64K  --.-KB/s    in 0s      

2018-02-26 10:28:52 (108 MB/s) - »logo_small.png« gespeichert [4749/4749]

                                                                                                                                                        [woffs@lapdoepp:/tmp/test] (master #%) $ cp logo_small.png lögö_smäll.png
[woffs@lapdoepp:/tmp/test] (master #%) $ git annex add
add logo_small.png ok
add lögö_smäll.png ok
(recording state in git...)
[woffs@lapdoepp:/tmp/test] (master +) $ git annex sync
commit 
adding metadata for logo_small.png
(recording state in git...)
[master (Basis-Commit) e3f8647] git-annex in woffs@lapdoepp:/tmp/test
 2 files changed, 2 insertions(+)
 create mode 120000 logo_small.png
 create mode 120000 "l\303\266g\303\266_sm\303\244ll.png"
ok

[woffs@lapdoepp:/tmp/test] (master) $ 
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

git annex is great. :)

> [[fixed|done]]; caused by git diff-index defaulting to not emitting such
> characters, got it to by using -z. --[[Joey]]