chainlint.sed: don't mistake `<< word` in string as here-doc operator

Tighten here-doc recognition to prevent it from being fooled by text
which looks like a here-doc operator but happens merely to be the
content of a string, such as this real-world case from t7201:

    echo "<<<<<<< ours" &&
    echo ourside &&
    echo "=======" &&
    echo theirside &&
    echo ">>>>>>> theirs"

This problem went unnoticed because chainlint.sed is not a real parser,
but rather applies heuristics to pretend to understand shell code. In
this case, it saw what it thought was a here-doc operator (`<< ours`),
and fell off the end of the test looking for the closing tag "ours"
which it never found, thus swallowed the remainder of the test without
checking it for &&-chain breakage.

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

t/chainlint.sed

@@ -89,6 +89,7 @@
 # here-doc -- swallow it to avoid false hits within its body (but keep the
 # command to which it was attached)
 /<<-*[ 	]*[\\'"]*[A-Za-z0-9_]/ {
+	/"[^"]*<<[^"]*"/bnotdoc
 	s/^\(.*\)<<-*[ 	]*[\\'"]*\([A-Za-z0-9_][A-Za-z0-9_]*\)['"]*/<\2>\1<</
 	s/[ 	]*<<//
 	:hered
@@ -100,6 +101,7 @@
 	s/^<[^>]*>//
 	s/\n.*$//
 }
+:notdoc
 
 # one-liner "(...) &&"
 /^[ 	]*!*[ 	]*(..*)[ 	]*&&[ 	]*$/boneline
@@ -151,8 +153,10 @@ s/.*\n//
 	/"[^'"]*'[^'"]*"/!bsqstr
 }
 :folded
-# here-doc -- swallow it
-/<<-*[ 	]*[\\'"]*[A-Za-z0-9_]/bheredoc
+# here-doc -- swallow it (but not "<<" in a string)
+/<<-*[ 	]*[\\'"]*[A-Za-z0-9_]/{
+	/"[^"]*<<[^"]*"/!bheredoc
+}
 # comment or empty line -- discard since final non-comment, non-empty line
 # before closing ")", "done", "elsif", "else", or "fi" will need to be
 # re-visited to drop "suspect" marking since final line of those constructs

t/chainlint/not-heredoc.expect

@@ -0,0 +1,14 @@
+echo "<<<<<<< ours" &&
+echo ourside &&
+echo "=======" &&
+echo theirside &&
+echo ">>>>>>> theirs" &&
+
+(
+	echo "<<<<<<< ours" &&
+	echo ourside &&
+	echo "=======" &&
+	echo theirside &&
+	echo ">>>>>>> theirs" ?!AMP?!
+	poodle
+) >merged

t/chainlint/not-heredoc.test

@@ -0,0 +1,16 @@
+# LINT: "<< ours" inside string is not here-doc
+echo "<<<<<<< ours" &&
+echo ourside &&
+echo "=======" &&
+echo theirside &&
+echo ">>>>>>> theirs" &&
+
+(
+# LINT: "<< ours" inside string is not here-doc
+	echo "<<<<<<< ours" &&
+	echo ourside &&
+	echo "=======" &&
+	echo theirside &&
+	echo ">>>>>>> theirs"
+	poodle
+) >merged