chainlint: colorize problem annotations and test delimiters

chainlint: colorize problem annotations and test delimiters

When `chainlint.pl` detects problems in a test definition, it emits the
test definition with "?!FOO?!" annotations highlighting the problems it
discovered. For instance, given this problematic test:

    test_expect_success 'discombobulate frobnitz' '
        git frob babble &&
        (echo balderdash; echo gnabgib) >expect &&
        for i in three two one
        do
            git nitfol $i
        done >actual
        test_cmp expect actual
    '

chainlint.pl will output:

    # chainlint: t1234-confusing.sh
    # chainlint: discombobulate frobnitz
    git frob babble &&
    (echo balderdash ; ?!AMP?! echo gnabgib) >expect &&
    for i in three two one
    do
    git nitfol $i ?!LOOP?!
    done >actual ?!AMP?!
    test_cmp expect actual

in which it may be difficult to spot the "?!FOO?!" annotations. The
problem is compounded when multiple tests, possibly in multiple
scripts, fail "linting", in which case it may be difficult to spot the
"# chainlint:" lines which delimit one problematic test from another.

To ameliorate this potential problem, colorize the "?!FOO?!" annotations
in order to quickly draw the test author's attention to the problem
spots, and colorize the "# chainlint:" lines to help the author identify
the name of each script and each problematic test.

Colorization is disabled automatically if output is not directed to a
terminal or if NO_COLOR environment variable is set. The implementation
is specific to Unix (it employs `tput` if available) but works equally
well in the Git for Windows development environment which emulates Unix
sufficiently.

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