check-non-portable-shell: improve `VAR=val shell-func` detection

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, thus should be avoided.

As such, check-non-portable-shell.pl warns when it detects such usage.
However, a limitation of the check is that it only detects such
invocations when variable assignment (i.e. `VAR=val`) is the first thing
on the line. Thus, it can easily be fooled by an invocation such as:

echo X | VAR=val shell-func

Address this shortcoming by loosening the check so that the variable
assignment can be recognized even when not at the beginning of the line.

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