Branch names with Unicode characters not correctly displayed

[Amedee Van Gasse]

**Steps to reproduce**

    git init
    git checkout -B feature/🦄

(copy/paste emoji from https://emojipedia.org/unicorn/)

Open repository in qgit (version 2.9-1build1 available from Ubuntu 20.04.1 LTS)

**Expected result**

Branch `feature/🦄` is properly displayed.

**Actual result**

`🦄` is replaced with a rectangle.

Also reported upstream: https://github.com/tibirna/qgit/issues/104

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$ apt-cache show qgit
Package: qgit
Architecture: amd64
Version: 2.9-1build1
Priority: optional
Section: universe/x11
Origin: Ubuntu
Maintainer: Ubuntu Developers <email address hidden>
Original-Maintainer: Andrey Rahmatullin <email address hidden>
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 1304
Depends: libc6 (>= 2.14), libgcc-s1 (>= 3.0), libqt5core5a (>= 5.12.2), libqt5gui5 (>= 5.7.0) | libqt5gui5-gles (>= 5.7.0), libqt5widgets5 (>= 5.11.0~rc1), libstdc++6 (>= 5), git
Suggests: source-highlight
Filename: pool/universe/q/qgit/qgit_2.9-1build1_amd64.deb
Size: 415500
MD5sum: c26e6708479d5f237c03f7fb359c42d0
SHA1: 7c28f37484ea0a7cb5eb6f5e35d03985b375acc5
SHA256: ee0bfac81f15d0375d222649e53d7f933f9ff9331da9288ac49039cd38b558ae
Homepage: https://github.com/tibirna/qgit
Description-en: Qt application for viewing GIT trees
 With qgit you will be able to browse revision tree, view patch content
 and changed files, graphically following different development branches.
 Main features:
  - View revisions, diffs, files history, files annotation, archive tree.
  - Commit changes visually cherry picking modified files.
  - Apply or format patch series from selected commits, drag and
    drop commits between two instances of qgit.
  - qgit implements a GUI for the most common StGIT commands like push/pop
    and apply/format patches. You can also create new patches or refresh
    current top one using the same semantics of git commit, i.e.
    cherry picking single modified files.
Description-md5: c8858a28936cee46b50707664d773e65

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$ sb_release -rd
Description: Ubuntu 20.04.1 LTS
Release: 20.04

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$ apt-cache policy qgit
qgit:
  Installed: 2.9-1build1
  Candidate: 2.9-1build1
  Version table:
 *** 2.9-1build1 500
        500 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        100 /var/lib/dpkg/status