perf: add test showing exponential growth in path globbing

perf: add test showing exponential growth in path globbing

Add a test showing that runtimes of the wildmatch() function used for
globbing in git grow exponentially in the face of some pathological
globs.

This issue affects both globs matching filenames via e.g. ls-files,
and globs matching refnames via e.g. for-each-ref.

As noted in the test description this is a test to see whether Git
suffers from the issue noted in an article Russ Cox posted today about
common bugs in various glob implementations:
https://research.swtch.com/glob

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