test: Add check-non-portable-shell.pl

test: Add check-non-portable-shell.pl

Add the perl script "check-non-portable-shell.pl" to detect
non-portable shell syntax.

"echo -n" is an example of a shell command working on Linux, but not
on Mac OS X.

These shell commands are checked and reported as error:

 - "echo -n" (printf should be used)
 - "sed -i" (GNUism; use a temp file instead)
 - "declare" (bashism, often used with arrays)
 - "which" (unreliable exit status and output; use type instead)
 - "test a == b" (bashism for "test a = b")

"make test-lint-shell-syntax" can be used to run only the check.

Helped-By: Jeff King <peff@peff.net>
Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>