test-lint: find unportable sed, echo, test, and export usage after &&

test-lint: find unportable sed, echo, test, and export usage after &&

Instead of anchoring these checks with "^\s*", just check that the
usage is preceded by a word boundary.  So now we can catch

	test $cond && export foo=bar

just like we already catch

	test $cond &&
	export foo=bar

As a side effect, this will detect usage of "sed -i", "echo -n", "test
a == b", and "export a=b" in comments.  That is not ideal but it's
potentially useful because people sometimes copy code from comments so
it can be good to also avoid nonportable patterns there.

To avoid false positives, keep the checks for 'declare' and 'which'
anchored.  Those are frequently used words in normal English-language
comments.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>