check-non-portable-shell: loosen one-shot assignment error message

When a0a6301 (t/check-non-portable-shell: detect "FOO=bar
shell_func", 2018-07-13) added the check for one-shot environment
variable assignment for shell functions, the primary reason given for
avoiding them was that, under some shells, the assignment outlives the
invocation of the shell function, thus could potentially negatively
impact subsequent commands in the same test, as well as subsequent
tests.

However, it has recently become apparent that this is not the only
potential problem with one-shot assignments and shell functions. Another
problem is that some shells do not actually export the variable to
commands which the function invokes[1]. More significantly, however, the
behavior of one-shot assignments with shell functions is not specified
by POSIX[2].

Given this new understanding, the presented error message ("assignment
extends beyond 'shell_func'") is too specific and potentially
misleading. Address this by emitting a less specific error message.

(Note that the wording "is not portable" is chosen over the more
specific "behavior not specified by POSIX" for consistency with almost
all other error message issued by this "lint" script.)

[1]: https://lore.kernel.org/git/[email protected]/
[2]: https://lore.kernel.org/git/[email protected]/

Signed-off-by: Eric Sunshine <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: sunshineco

t/check-non-portable-shell.pl

@@ -50,7 +50,7 @@ sub err {
 	/\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
-		err '"FOO=bar shell_func" assignment extends beyond "shell_func"';
+		err '"FOO=bar shell_func" is not portable';
 	$line = '';
 	# this resets our $. for each file
 	close ARGV if eof;