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>

Author: sunshineco

t/check-non-portable-shell.pl

@@ -49,7 +49,7 @@ sub err {
 	/\bexport\s+[A-Za-z0-9_]*=/ and err '"export FOO=bar" is not portable (use FOO=bar && export FOO)';
 	/\blocal\s+[A-Za-z0-9_]*=\$([A-Za-z0-9_{]|[(][^(])/ and
 		err q(quote "$val" in 'local var=$val');
-	/^\s*([A-Z0-9_]+=(\w*|(["']).*?\3)\s+)+(\w+)/ and exists($func{$4}) and
+	/\b([A-Z0-9_]+=(\w*|(["']).*?\3)\s+)+(\w+)/ and !/test_env.+=/ and exists($func{$4}) and
 		err '"FOO=bar shell_func" is not portable (use test_env FOO=bar shell_func)';
 	$line = '';
 	# this resets our $. for each file