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>