chainlint.pl: don't require return|exit|continue to end with &&

In order to check for &&-chain breakage, each time TestParser encounters
a new command, it checks whether the previous command ends with `&&`,
and -- with a couple exceptions -- signals breakage if it does not. The
first exception is that a command may validly end with `||`, which is
commonly employed as `command || return 1` at the very end of a loop
body to terminate the loop early. The second is that piping one
command's output with `|` to another command does not constitute a
&&-chain break (the exit status of the pipe is the exit status of the
final command in the pipe).

However, it turns out that there are a few additional cases found in the
wild in which it is likely safe for `&&` to be missing even when other
commands follow. For instance:

    while {condition-1}
    do
        test {condition-2} || return 1 # or `exit 1` within a subshell
        more-commands
    done

    while {condition-1}
    do
        test {condition-2} || continue
        more-commands
    done

Such cases indicate deliberate thought about failure modes by the test
author, thus flagging them as breaking the &&-chain is not helpful.
Therefore, take these special cases into consideration when checking for
&&-chain breakage.

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

t/chainlint/return-loop.test

@@ -0,0 +1,6 @@
+while test $i -lt $((num - 5))
+do
+# LINT: "|| return {n}" valid loop escape outside subshell; no "&&" needed
+	git notes add -m "notes for commit$i" HEAD~$i || return 1
+	i=$((i + 1))
+done