chainlint: add test of pathological case which triggered false positive

chainlint: add test of pathological case which triggered false positive

This extract from contrib/subtree/t7900 triggered a false positive due
to three chainlint limitations:

* recognizing only a "blessed" set of here-doc tag names in a subshell
  ("EOF", "EOT", "INPUT_END"), of which "TXT" is not a member

* inability to recognize multi-line $(...) when the first statement of
  the body is cuddled with the opening "$("

* inability to recognize multiple constructs on a single line, such as
  opening a multi-line $(...) and starting a here-doc

Now that all of these shortcomings have been addressed, turn this rather
pathological bit of shell coding into a chainlint test case.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>