### Please describe the problem.
I had also commented this on [[another bug|bugs/git-annex_fromkey_barfs_on_utf-8_input]], but the original issue there is fixed now.
I tested `fromkey`, `calckey --batch`, `lookupkey --batch` (in standalone) after your fix, they work nicely.

However, `git-annex metadata --batch --json` using the [[linux standalone|install/Linux_standalone]] (autobuild) still fails when it encounters UTF-8 characters (e.g. ü, ç, ä).
Also, `git-annex metadata --json` gives `"file":"��.txt"` for `ü.txt`.

This happens only in the standalone builds.

### What steps will reproduce the problem?

[[!format sh """
$ .../git-annex.linux/runshell
$ touch u.txt ü.txt
$ git-annex add .

$ git-annex metadata --batch --json
{"file":"ü.txt"}
git-annex: Batch input parse failure: Error in $: Failed reading: Cannot decode byte '\xb3': Data.Text.Internal.Encoding.decodeUtf8: Invalid UTF-8 stream

$ git-annex metadata --batch --json
{"file":"u.txt","fields":{"ç":["b"]}}
git-annex: Batch input parse failure: Error in $: Failed reading: Cannot decode byte '\xb3': Data.Text.Internal.Encoding.decodeUtf8: Invalid UTF-8 stream

$ git-annex metadata --batch --json
{"file":"u.txt","fields":{"b":["ä"]}}
git-annex: Batch input parse failure: Error in $: Failed reading: Cannot decode byte '\xb3': Data.Text.Internal.Encoding.decodeUtf8: Invalid UTF-8 stream

$ git-annex metadata --json
{"command":"metadata","note":"","success":true,"key":"SHA256E-s0--e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855.txt","file":"u.txt","fields":{}}
{"command":"metadata","note":"","success":true,"key":"SHA256E-s0--e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855.txt","file":"��.txt","fields":{}}
# success, but the second line should have "file":"ü.txt"
"""]]

It's the same even if I call `.../git-annex.linux/git-annex` directly (without `runshell`)

### What version of git-annex are you using? On what operating system?
Using the Linux standalone: [git-annex-standalone-amd64.tar.gz](https://downloads.kitenet.net/git-annex/autobuild/amd64/git-annex-standalone-amd64.tar.gz) on Xubuntu 16.04

[[!format sh """
$ git-annex version
git-annex version: 6.20161213-g55a34b493
build flags: Assistant Webapp Pairing Testsuite S3(multipartupload)(storageclasses) WebDAV Inotify DBus DesktopNotify XMPP ConcurrentOutput TorrentParser MagicMime Feeds Quvi
key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_224 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 SHA1E SHA1 MD5E MD5 WORM URL
remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav tahoe glacier ddar hook external
local repository version: 5
supported repository versions: 3 5 6
upgrade supported from repository versions: 0 1 2 3 4 5
operating system: linux x86_64
"""]]

### Please provide any additional information below.

None of the characters I used have `\xb3` in it, but all the errors happen due to it:
[[!format sh """
$ .../git-annex.linux/runshell
$ echo -n ü | xxd
00000000: c3bc                                     ..
$ echo -n ç | xxd
00000000: c3a7                                     ..
$ echo -n ä | xxd
00000000: c3a4                                     ..
"""]]

In `runshell`, `ls` can't show UTF-8, but `git-annex status` can:
[[!format sh """
$ .../git-annex.linux/runshell
$ ls
u.txt  ??.txt
$ git-annex status
A u.txt
A ü.txt
"""]]

`man` complains about locale in `runshell` as well:
[[!format sh """
$ .../git-annex.linux/runshell
$ man
man: can\'t set the locale; make sure $LC_* and $LANG are correct
What manual page do you want?
# I escaped that \', formatting was messy otherwise
$ set | grep LANG
GDM_LANG='en_GB'
LANG='en_GB.UTF-8'
LANGUAGE='en_GB:en'
$ set | grep LC
# nothing
"""]]