t/check-non-portable-shell: detect "FOO= shell_func", too

t/check-non-portable-shell: detect "FOO= shell_func", too

Just like assigning a nonempty value, assigning an empty value to a
shell variable when calling a function produces non-portable behavior:
in some shells, the assignment lasts for the duration of the function
invocation, and in others, it persists after the function returns.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>