chainlint: add explanatory comments

The logic in TestParser::accumulate() for detecting broken &&-chains is
mostly well-commented, but a couple branches which were deemed obvious
and straightforward lack comments. In retrospect, though, these cases
may give future readers pause, so comment them, as well.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

t/chainlint.pl

@@ -505,7 +505,11 @@ my @safe_endings = (
 
 sub accumulate {
 	my ($self, $tokens, $cmd) = @_;
+
+	# no previous command to check for missing "&&"
 	goto DONE unless @$tokens;
+
+	# new command is empty line; can't yet check if previous is missing "&&"
 	goto DONE if @$cmd == 1 && $$cmd[0] eq "\n";
 
 	# did previous command end with "&&", "|", "|| return" or similar?