chainlint: make error messages self-explanatory

The annotations emitted by chainlint to indicate detected problems are
overly terse, so much so that developers new to the project -- those who
should most benefit from the linting -- may find them baffling. For
instance, although the author of chainlint and seasoned Git developers
may understand that "?!AMP?!" is an abbreviation of "ampersand" and
indicates a break in the &&-chain, this may not be obvious to newcomers.

The "?!LOOP?!" case is particularly serious because that terse single
word does nothing to convey that the loop body should end with
"|| return 1" (or "|| exit 1" in a subshell) to ensure that a failing
command in the body aborts the loop immediately. Moreover, unlike
&&-chaining which is ubiquitous in Git tests, the "|| return 1" idiom is
relatively infrequent, thus may be harder for a newcomer to discover by
consulting nearby code.

Address these shortcomings by emitting human-readable messages which
both explain the problem and give a strong hint about how to correct it.

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

t/chainlint/nested-subshell.expect

@@ -7,7 +7,7 @@
 8 
 9 	cd foo &&
 10 	(
-11 		echo a ?!AMP?!
+11 		echo a ?!LINT: missing '&&'?!
 12 		echo b
 13 	) >file
 14 )