t/perf: correctly align non-ASCII descriptions in output

t/perf: correctly align non-ASCII descriptions in output

Change the test descriptions from being treated as binary blobs by
perl to being treated as UTF-8. This ensures that e.g. a test
description like "æ" is counted as 1 character, not 2.

I have WIP performance tests for non-ASCII grep patterns on another
topic that are affected by this.

Now instead of:

    $ ./run p0000-perf-lib-sanity.sh
    [...]
    0000.4: export a weird var                                    0.00(0.00+0.00)
    0000.5: éḿíẗ ńöń-ÁŚĆÍÍ ćḧáŕáćẗéŕś   0.00(0.00+0.00)
    0000.7: important variables available in subshells            0.00(0.00+0.00)
    [...]

We emit:

    [...]
    0000.4: export a weird var                                 0.00(0.00+0.00)
    0000.5: éḿíẗ ńöń-ÁŚĆÍÍ ćḧáŕáćẗéŕś                          0.00(0.00+0.00)
    0000.7: important variables available in subshells         0.00(0.00+0.00)
    [...]

Fixes code originally added in 342e9ef2d9 ("Introduce a performance
testing framework", 2012-02-17).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>