`git add` completion fails when using filenames with Scandinavian characters

== Background ==

Reporting from Xenial, but this is currently reproducible in Artsy too.

$ locale
LANG=fi_FI.UTF-8
LANGUAGE=fi:en
LC_CTYPE=fi_FI.UTF-8
LC_NUMERIC="fi_FI.UTF-8"
LC_TIME="fi_FI.UTF-8"
LC_COLLATE=fi_FI.UTF-8
LC_MONETARY="fi_FI.UTF-8"
LC_MESSAGES=fi_FI.UTF-8
LC_PAPER="fi_FI.UTF-8"
LC_NAME="fi_FI.UTF-8"
LC_ADDRESS="fi_FI.UTF-8"
LC_TELEPHONE="fi_FI.UTF-8"
LC_MEASUREMENT="fi_FI.UTF-8"
LC_IDENTIFICATION="fi_FI.UTF-8"
LC_ALL=

== Test case ==

$ git init test
$ cd test
$ echo "foo" > bär
$ git add [hit tab key]
$ git add \"b\\303\\244r\" [hit enter]

== What happens ==

fatal: pathspec '"b\303\244r"' did not match any files

== What I expect to happen ==

For the filename to be correctly completed, like with ls:

$ mkdir test2
$ cd test2
$ echo "foo" > bär
$ ls [hit tab key]
$ ls bär [hit enter]
bär

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: bash-completion 1:2.1-4.2ubuntu1.1 [modified: usr/share/bash-completion/completions/apt]
ProcVersionSignature: Ubuntu 4.8.0-54.57~16.04.1-generic 4.8.17
Uname: Linux 4.8.0-54-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.6
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Jun 5 18:37:14 2017
InstallationDate: Installed on 2016-10-13 (235 days ago)
InstallationMedia: Ubuntu-Server 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
PackageArchitecture: all
SourcePackage: bash-completion
UpgradeStatus: No upgrade log present (probably fresh install)