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>

t/chainlint/t7900-subtree.expect

@@ -0,0 +1,10 @@
+(
+	chks="sub1sub2sub3sub4" &&
+	chks_sub=$(cat | sed 's,^,sub dir/,'
+>>) &&
+	chkms="main-sub1main-sub2main-sub3main-sub4" &&
+	chkms_sub=$(cat | sed 's,^,sub dir/,'
+>>) &&
+	subfiles=$(git ls-files) &&
+	check_equal "$subfiles" "$chkms$chks"
+>)

t/chainlint/t7900-subtree.test

@@ -0,0 +1,22 @@
+(
+	chks="sub1
+sub2
+sub3
+sub4" &&
+	chks_sub=$(cat <<TXT | sed 's,^,sub dir/,'
+$chks
+TXT
+) &&
+	chkms="main-sub1
+main-sub2
+main-sub3
+main-sub4" &&
+	chkms_sub=$(cat <<TXT | sed 's,^,sub dir/,'
+$chkms
+TXT
+) &&
+
+	subfiles=$(git ls-files) &&
+	check_equal "$subfiles" "$chkms
+$chks"
+)