Full test case

# install current git version (2.8.3)
# use only default options

# open git bash

# show some properties
jo@eld MINGW64 /t/tmp
$ uname
MINGW64_NT-6.1

jo@eld MINGW64 /t/tmp
$ git --version
git version 2.8.3.windows.1

jo@eld MINGW64 /t/tmp
$ echo $LANG
de_DE.UTF-8

# remove global gitconfig
jo@eld MINGW64 /t/tmp
$ mv ~/.gitconfig  ~/.gitconfig.org.$(date +'%Y%m%dT%H%M%S')

# create new repository
jo@eld MINGW64 /t/tmp
$ git init test
Initialized empty Git repository in T:/tmp/test/.git/

jo@eld MINGW64 /t/tmp
$ cd test

# set encoding to utf-8, users name and email
# and show some file properties and git config
jo@eld MINGW64 /t/tmp/test (master)
$ git config gui.encoding utf-8

jo@eld MINGW64 /t/tmp/test (master)
$ file .git/config
.git/config: ASCII text

jo@eld MINGW64 /t/tmp/test (master)
$ git config user.name Groß

jo@eld MINGW64 /t/tmp/test (master)
$ git config user.email gross@example.com

jo@eld MINGW64 /t/tmp/test (master)
$ file .git/config
.git/config: UTF-8 Unicode text

jo@eld MINGW64 /t/tmp/test (master)
$ git config --list
core.symlinks=false
core.autocrlf=input
core.fscache=true
color.diff=auto
color.status=auto
color.branch=auto
color.interactive=true
help.format=html
http.sslcainfo=C:/Program Files/Git/mingw64/ssl/certs/ca-bundle.crt
sendemail.smtpserver=/bin/msmtp.exe
diff.astextplain.textconv=astextplain
rebase.autosquash=true
credential.helper=manager
core.repositoryformatversion=0
core.filemode=false
core.bare=false
core.logallrefupdates=true
core.symlinks=false
core.ignorecase=true
core.hidedotfiles=dotGitOnly
gui.encoding=utf-8
user.name=Groß
user.email=gross@example.com

# commit a change
jo@eld MINGW64 /t/tmp/test (master)
$ echo 'Test file' > t.txt

jo@eld MINGW64 /t/tmp/test (master)
$ git add t.txt

jo@eld MINGW64 /t/tmp/test (master)
$ git commit -m 'a test file' -m 'Groß'
[master (root-commit) 7cdf009] a test file
 1 file changed, 1 insertion(+)
 create mode 100644 t.txt

jo@eld MINGW64 /t/tmp/test (master)
$ git log
commit 7cdf00988023305d1701dcde5a8e27dfc91a49c3
Author: Groß <gross@example.com>
Date:   Sat May 21 15:43:09 2016 +0200

    a test file

    Groß

# amend the commit
jo@eld MINGW64 /t/tmp/test (master)
$ git commit --amend -m 'A test file' -m 'Groß'
[master 95d73cc] A test file
 Date: Sat May 21 15:43:09 2016 +0200
 1 file changed, 1 insertion(+)
 create mode 100644 t.txt

jo@eld MINGW64 /t/tmp/test (master)
$ git log
commit 95d73cc05de08a54e4659e76ce929de802ec6424
Author: Groß <gross@example.com>
Date:   Sat May 21 15:43:09 2016 +0200

    A test file

    Groß

# open git gui
# and amend the commit
jo@eld MINGW64 /t/tmp/test (master)
$ git gui

# show commit
# author name is destroyed 
jo@eld MINGW64 /t/tmp/test (master)
$ git log
commit ce95aa942ad9330baebfee2dea3889f14043b7de
Author: GroÃ? <gross@example.com>
Date:   Sat May 21 15:43:09 2016 +0200

    A test file / Amend Last Commit in Git Gui

    Groß

jo@eld MINGW64 /t/tmp/test (master)
$