t/perf: avoid unnecessary test_export() recursion

t/perf: avoid unnecessary test_export() recursion

test_export() has been self-recursive since its inception even though a
simple for-loop would have served just as well to append its arguments
to the `test_export_` variable separated by the pipe character "|".
Recently `test_export_` was changed instead to a space-separated list of
tokens to be exported, an operation which can be accomplished via a
single simple assignment, with no need for looping or recursion.
Therefore, simplify the implementation.

While at it, take advantage of the fact that variable names to be
exported are shell identifiers, thus won't be composed of special
characters or whitespace, thus simple a `$*` can be used rather than
magical `"$@"`.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>