t3430: drop unnecessary one-shot "VAR=val shell-func" invocation

t3430: drop unnecessary one-shot "VAR=val shell-func" invocation

The behavior of a one-shot environment variable assignment of the form
"VAR=val cmd" is unspecified according to POSIX when "cmd" is a shell
function. Indeed the behavior differs between shell implementations and
even different versions of the same shell. One such problematic behavior
is that, with some shells, the assignment will outlive the invocation of
the function, thus may potentially impact subsequent commands in the
test, as well as subsequent tests. A common way to work around the
problem is to wrap a subshell around the one-shot assignment, thus
ensuring that the assignment is short-lived.

In this test, the subshell is employed precisely for this purpose; other
side-effects of the subshell, such as losing the effect of `test_tick`
which is invoked by `test_commit`, are immaterial.

These days, we can take advantage of `test_commit --author` to more
clearly convey that the test is interested only in overriding the author
of the commit.

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